That happens a lot when you move on with your own life after a time.
In her case, it sounds like she is one of those people who will drop all of her friends for that special someone. You also have grown apart because you both have your own lives and have new people and earned new friends.
I have had that happen in my case a few times. In fact, my other friends and I are going through this situation with what we thought was a friend. Like your situation, he is suddenly ignoring us for his special lady that he has been wanting for a long time.
Anyway, what I had started doing is picking up on certain cues while associating with the person at the time and then reflect back on those. (Body language, facial expressions, things that are said and how they are said, how they treat your other friends and other people)