can anyone help please?? :-(

My girlfriend is normally very affable & friendly.we are together for more than 4 months.We've been friends for 2 years.We have known each other pretty well..She had a lot of male friends..some of them are really close.One of them is studying with her.I don't know that guy.He is a close friend of her.They have a good communication.Few months ago,he gave her a gift,a watch,for her birthday & she wears with now.I met her three times since then and was wearing that watch,even on our date.That got me sick.Even the profile picture on facebook she has right now was taken by him.They chat on whats app,sometimes the guy calls her & keeps talking for a long time.She doesn't have other close guy friends there.She takes notes from him and sometimes they meet together in the college.I forgot to mention I am in another college.Before our relationship,they participated in a dance performance together.And some in their college thought they're couples.She said she feels bad that everybody thinks alike.She shared all these things with me,but i didn't now about her this much of intimacy with that guy before relation.We had a fight about this guy once when i got angry with her about their intimacy.She was crying.She became sad & said she was upset seeing me not trusting her how to keep distance from him.We text and call each other a lot.When for some reason any of us do not reply the other one goes a bit worried.She loves me a lot & i know that but these things get me sick.What should i do?