My grandpa had this one show up on his cam , i think it is just a big cyst but i wonder if she could have had a still born fawn and she never aborted it .
I would say cyst. Looks like infection from unborn fawn would have killed her by now. Sad to see, but she seems to be making it, at least up until now.
My bet is it is a fawn that is trapped in there. the body will kind of "mummify" it in its own sack... nasty deal... if its my I would have a high likleyhood of SSS...
I'm sure someone with more experience than me could tell you, but it looks like a cyst or tumor. In times of stress I've heard doe will 'assimilate' or abort the fawn. This is way beyond that.