How's it going, everyone? First of all, english is not my first language go easy on me. I'll call myself Hoyt.
I also apologize for the post size. It's a good story, though, you should read it
There will be a TL;DR at the end, for your convenience.

I'm 24M, in a serious relationship (she's also 24).
We met a couple of years ago (2016), because of a common friend (let's call her Linda), but she was in a serious relationship at the time. She broke up with her boyfriend soon after, and we started talking to each other around oct/18. I told the mutual friend I was planning on asking her out, and that friend advised me not to. She said that her friend (my current girlfriend - let's call her Jolene) was seeing some other guy, and things were getting serious. At that point in time I didn't have any sort of contact with her at all (I'm not active on social media).
By the time april was coming to an end, Linda told me she was not seeing that guy anymore, and I should ask her out. I didn't have the time to do so (my father had died 2 months earlier, and I was not in the mood), because she asked me out the very next day. We made plans for lunch and a movie (we both have very busy schedules), and things started very well. On our second date - 3 days later - I asked her about past relationships (as she asked about mine). She told me about her ex, whom was very toxic for her. Later on, I asked about the guy she was seeing before we started going out, and she told me they stopped seeing each other about a month earlier.
I asked her to be my girlfriend 18 days after our first date, and she said yes. I should point out that we went on about 10 dates in 2 those "almost" 3-weeks, before we dove into a commited relationship. Briefly after we started dating, she told me about a party she went to (right after our second date - I knew she was there because she texted me the whole night), and kissed another guy. I didn't like it very much but after thinking about it for a while, I made my peace with it. We were not in a commited relationship, and it was just a kiss with some guy she never saw again. I should mention my birthday was 3 days before we decided to take things seriously, and 1 day after we had sex for the first time.
Fast forward to september. We were about 4 months in as a couple. We were at a bar, and we started talking about past relationships again (in a playful way). But she told me something, and I started 'counting' mentally on the calendar, and things didn't add up. I pushed her a little and she confessed me the last time she saw the guy she was going out with (i.e had sex), before going out with me, was actually 2 days before we went out, instead of >1mo. I didn't like it at all (mostly 'cause she was capable of keeping a lie for that long). She cried for a while, but we were ok after all. I should point out something else. We always used protection, for our entire relationship (to this day - and I mean it, we NEVER did anything wrong), and she was always paranoid about being pregnant. She latter confessed me she didn't use any protection while with him.
One day later, i forwarded some photos from her phone to mine (I took them with her phone simply cause it has a better camera). Later the same day, I accidentally deleted one of those photos, and needed her phone to re-send it to myself. My fingerprint is registered, so I had no trouble unlocking it. As soon as I did it, I saw a message from Linda (my so called friend), it said something like:

You shouldn't be worried about a unexpected pregnancy. If you're actually pregnant, it's Hoyt's. You can't be pregnant for months and not noticing it.

I freaked out and opened the conversation.
She confessed she had sex with the last guy she went out with, while we were together. 3 days before we decided to take things seriously, and 1 day after we had sex for the first time. On my birthday. Her text to my "friend" was something like this:

Hoyt can never find out about this, but the last time I had sex with the other guy was actually on is birthday.

I almost broke up with her immediately. It was the worst birthday of my life. Me and my father were very close, and it was my first birthday without him. I couldn't see her at the day (I went to visit my family that day - though nobody showed up besides my brother and my mother), but I still had the texts from that day. She texted me the whole day & night, including when she was out with that guy (I knew she had gone out, she told me before going). Sent a lot of voice mails wishing happy birthday, and telling me she missed me a lot. As I think about it, it was one of the days I realized I really liked her. She suddenly stopped texting at 11pm, and 1am sent a text like "I just got home and I'm going to bed, xo".
All of this happened 2 weeks ago. I told her I'm okay with it right now, just to make her stop bothering me, asking me what I'm gonna do. I still don't know. She cried a lot, apologized and told me that was the last time. She also told she did it because she "needed to know she didn't have any more feelings for him".
We had plans of moving in together, and marring each other (she thinks we still do). Despite all of this, she was very supporting. I went through some terrible stuff in all of the aspects of my life in the first 2 months of our relationship, and she was by my side at all times.
Nontheless, I'm very hurt. She kept that from me for a very long time, with a straight face. I’m having trouble believing that was the last time she saw him: all of the following months she kept freaking out about being pregnant. And I don’t have any means of finding out (she deleted everything from her phone 1 month in our relationship).
I’m also second guessing her character, and how much she likes me. She had only 1 boyfriend before me. From the moment they hooked up, she never even kissed another guy (and they were in a long distance relationship, going through a month without seeing each other). Same thing happened with the guy before me. She kissed other people, but never had sex with someone else while with him. I found a post on social midia (by her) where she asked for advice on “keeping a guy that doesn’t want a serious relationship”, mentioning the guy she ‘cheated’ on me with.
I also was never intimidated by that guy. She complained he’s 30 and lives with his parents, makes no money at all, he’s substancially worst looking than I am, and she claims our sexual chemistry is better (and I do believe it - she's the best I ever had).
Don't know what to do from here. I still like her. And I believe she likes me. But I can't stand thinking about that day, and keep doubting she's a "keeper", based on the fact she had sex with another guy, on my birthday, while we were seeing each other (and things were going well, or at least I thought so).
Am I overreacting? Should I end things now? What would you do?

TL;DR:
My girlfriend had sex with some guy she went out with earlier that year, regularly, while we were dating, on my birthday, and now I'm doubting her character and second-guessing our entire relationship.

Sorry for the bible.
Thanks for making it through.