You are not going to get any proper answers on here, even from a vet, because we can't see the dog or the lump - which we need to make any form of diagnosis.
You're best taking her down to your vet and find out what it is - that way if it is anything nasty the vet can do something about it when there's still time.
Differentials include abscess, haematoma (accumulation of blood), seroma (accumulation of fluid) and lipoma (benign overgrowth of fat cells in one particular area.). Lipomas in particular can grow quite large. Nothing is usually done about them unless they are bothering the dog or interfering with movement because they aren't too serious.
If anything is done that is painful, the vet will prescribe painkillers. We take an oath to do the best for the welfare of the animal presented to us - not to make it worse or make it suffer. The most likely thing the vet will do initially is a Fine Needle Aspirate - where you take a needle and put it into the mass, and try and suck some cells up to see what type they are. It hurts about as much as in injection.