Why do worry about my girlfriend?!?

Ok so me and my girlfriend have been dating for almost 2 years now and just recently i have been worrying like when she is out with a group of friends that includes guys or even if like im at work and she isnt texting back or something. And she hasnt given me a reason not to trust her other than a couple ofguys texting her but that isnt enough. So my question is why do i worry when my girlfriend goes out with friends or not talking back to me? i feel like i almost dont trust her for no apparent reason? Please any advice would be great! thanks.