Hey buddy, my situation is very similar to yours. Feel free to read my thread, 23 comments and they have helped me a lot.
her-ex-still-in-the-picture-how-to-deal ... 63835.html
With your situation I would make a decision and stick with it. Either you really dont trust her (your gut u said) or you are able to move past this and continue (make a clean break from feeling insecure about the other guy). Its either 1 or the other. If you will always have this "lingering" feeling she may be going behind your back... DROP her NOW. BC itll always be there and girls HATE guys who are insecure with THEMSELVES. And you will always be doubting... scared... ect. Dont live like that. You deserve better.
The evidence in my situation and yours is pretty harmless IMO. If you knew for sure they were seeing each other like every day and hanging out 1 on 1 every day your not... then id drop her. But if she says they just talk then its whatever... he may be a big jock athlete... but hes a fucking tool probably.. doesnt understand game like you do and ultimately you will win her by UNDERSTANDING what is ATTRACTIVE to girls.
You sound young and so does she... so you should have 5-6 girls your "orbiting" as well incase this main chick fades. THIS ISNT CHEATING on your part or hers... you just havnt even come close to developing a SOLID relationship with each other...."Yet". Until then (it takes time...) go out and meet new bitches 2... and what ull see happen is it wont bother you if she hangs out or talks to other guys.... BC ull be gaming and owning other girls.... which will DRIVE HER to BEG for YOUR attention. Thats how you win this game sir. SOCIAL PROOF.
Take this as a LEARNING experience. You have oneitis a little bit, but thats ok. Better YOURSELF and the results around you will manifest. Dont try and change her or the situation.... but change YOU!
GL
DUKE