I'm assuming "she" is your girlfriend? Well, not knowing any other details of your situation (which would be really helpful if you are willing to share) I could give you a few theories.
1) She is insecure with herself and is intimidated by you being around other women.
2) She doesn't know your friends well and feels threatened by their relationship with you because you keep them "separate".
3) Something has happened (like cheating) in your relationship that has made her lose trust in you.
4) You don't allow her to have male friends so she feels you must be up to no good with your female friends
I could add a lot more to this list but it seems silly without any more information on your end. So if you're wiling to share I am willing to give my thoughts.