Unwarranted nervousness/anxiety about girlfriend

Me and this girl have been pretty much best friends for the past 3 years. Its always been mutually platonic though. However, we both went to college and ended up at the same school. She's a sophomore, I'm a freshman. Well once I got to college and we started hanging out again, something changed. We both started to like each other. At first it we didn't do anything about it because we didn't want to ruin the awesome friendship we have. But it kept progressing and eventually we got to the point where we said we have to give a relationship a shot or just stop doing this altogether. We both decided that we weren't ready for a relationship so we decided to stop. Well it wrecked both of us. We were both super sad and just felt like trash. We only lasted about a week before we decided maybe we should try a relationship. Its been awesome. I really like her and she's said the same to me. Its been a pretty stressful school year and pretty much my only true happy time is when I am with her. Ive made a bunch of other really close friends here too so that's been really cool. The problem is I get super anxious and nervous whenever my girlfriend hangs out with her other friends. I completely trust her, I know she won't cheat or anything like that. I know her friends quite well, they're also some of my close friends and I also trust them. Theres absolutely no reason for me to get anxious about it. She's just living her life the same as I am mine. But this nervousness has just kept getting worse and worse and at this point its constant and flares up even harder when she goes and hangs out with people. I just want it to stop because Its hurting me and there's no good reason for it. Any suggestions what might be causing it/what I can do to help it?