This could be serious and she needs a vet. You don't know what this is and at this point, it seems only a vet can tell. If it an abscess, the chances of it popping itself are slim. An abscess can kill an animal if the puss gets into the bloodstream (that's how my rat died). An abscess is basically a puss filled sack if you did not know. If it is an abscess, a vet can pop it safely. However, if it a broken tooth, or some kind of tumor, she REALLY needs a vet. She could be in pain and it's not good to let her live like this. I may sound pushy, mean, and overreactive, but I am just telling the truth.