It could be a cyst or fatty lump. Does it move about when you touch it and the skin attached to it or is it stationary? Hard or soft to the touch? She hasn't been bitten there or anything?
Mimzy had a fatty lump removed from his left side/armpit area a few years ago. I was told they show up for no apparent reason on rabbits and can either be left alone or removed. They are usually benign.
I'm hoping that is all it is. ((((((((((((Huge hugs and 'nothing sinister' vibes)))))))))) Hang in there, hun. I will be thinking of you all today. xxxx