The problem you never thought you had feelings for her is because you had her to yourself whenever you wanted her for the most part, now that she is seeing someone else you know that it wont be that way anymore because she will be spending a lot more time with her man. This is probably gonna suck pretty bad for you, since you got use to being around her so much.
The only thing I can tell you is keep being the friend that you have been, and if something happens be there for her. Then if you get the chance next time try not to let it pass you by.
Good Luck.