have not been in this situation before... need help!

Then what? The thing is.. is she still having sex with that guy? It is this that is bothering me. Because if they do I won’t be with her anymore. Who wants a girl who is sleeping with other guys whenever both wants to?

I could ask her but then she would get mad at me for not trusting her.

Unless you've both discussed and agreed upon exclusivity, it's none of your business who she has sex with. Apparently it's not you so... maybe you need just to find some one else.

the person you need to talk to about this is the young woman/girl

if either of you isn't up to the conversation, you and the relationship may not be mature enough to continue

try talking to her as the adult you are supposed to be

this may be your answer

you need a more adult partner - someone you can talk to about things

Yes but I had time to think and both neptune and you tell the truth. It is none of my business. Now I'm afraid of losing her cause at first when she was teasing me I thought she did that just to cuddle, but when I look back it could have been a sign that she wanted to get laid and I didn't catch up on that. So the chances are that she is looking for a another guy because she might have taken it as me not being interested in her.

How can I trust her?

I would only do this turn on to make you jealous and more interested in making me your exclusive squeeze. since she was with a guy that did her before she was testing your manhood.