Jealousy is a choice, it's an emotion but you choose to react to it.   If you are jealous then it's because something else is amiss not their friendship.  I've been on the other side where I had many platonic friends who were women.  Eventually what happened over time is my GF now wife and I started hanging out with the ones she ended up having her own friendship with and it became it's own thing.

I guess for you, do you feel like the third wheel?  If so I'd say there's a chance your GF is finding something outside of the relationship she's needing that is not there.  That could be an easy fix or simply something that can never exist.  

If I'm you I lay out the feeling, that you don't want to feel jealous.  Ask her if there's something she misses/wants that not in your relationship and go from there.  The point here is if there is something and either you or she can't get over it, then it's better to know now.