This is my view

Because you have been away so long you need to know for certain that she isn't seeing anyone else. So just be prepared here as she may not have the courage to tell you yet, hence this could be the  reason she doesn't feel the same about you  as before .A lot can happen in 3 months.

If that situation is clear , then perhaps you just need to reignite the spark between you . This is not dissimilar to problems in longer term relationships including marriages  when the relationship goes stale . A good tip is to start going to places where you have gone together  in the past and put a little romance into the proceedings . Doing more things together will also help .Perhaps go away together on a mini break .