girlfriend cheating...?.

ahhh where do I start well lets go from here my girlfriend went to germany by herself she is 17 by the way. and stayed up there with some guy friends. after she has come back she isnt the same it seems. she wont tell me much of her trip, doesnt say I love you much and well just more like friends when we hang out really. all I’ve heard of her trip was that she went to german discos in dusseldorf. and she got drunk every night with her friends. She gets mad if I ask her if anything happened and I really am worried if something might have happened over there. she knows those people over there really well since she lived there from age 4 to 14 so it wasnt like she didnt know anyone. and the majority of the people she stayed with were all guys. the only reason I know they were guys is at last minute before she got on the plane I found out the the girl she was going to stay with had other plans so a guy friend said she could stay with him. I dont know him or anything of him and it makes me mad. Other than going to clubs and discos I havent heard much and worried something had to of happened over there. I just want anyones input on this and I can give more detail if you have a question or if I left something out

to add more to this two weeks before she left to germany she was at her cousins who is in college a senior and her friends. reason I say this is because while she was up there she has pictures of guys on her phone and camera. one where she is sleeping in bed with another guy( both wernt under covers and they had clothes on) she insists that her cousin had them jump on her while she took the picture but I dont know…they looks relaxed in the picture to me he was basically spooning her with clothes on to be short…so ya thats my story of it and she still talks to that guy on the phone sometimes