feel her gum to see if they are hardening if they are that means she is starting to teeth, you are definitely her comfort when it comes to her mouth being put anywhere; so she feels comfortable with biting down on you.
I read further down in this group someone had the same problem, like you say no and ouch and if she does it again, you stop the session for the moment and try again later. It may take time, but she will figure it out!