How do you deal with feeling threatened?

Not sure I'm in just being ridiculous and insecure but...

I've been feeling a little threatened/super uncomfortable when my SO studies with his female study partner. I don't usually have a problem with him spending time with his female friends but his girl has a reputation of sleeping with her study partners and has gone around most of the cohort already. I've met her, she seemed ok. I later learned she coined the nickname 'miss piggy' for me which my SO started to call me too 😭 (quickly addressed that one). She doesn't respect that my SO has a girlfriend and he frequently tells me she makes him uncomfortable too but he still continues to study with her. I brought up how uncomfortable I felt last night and he assured me there's nothing to worry about. I trust him completely. I do NOT trust HER. I don't know what to do. Am i being ridiculous? Should I just keep my mouth shut and trust him? Maybe I'm just feeling insecure? Your thoughts would be much appreciated ☺️