Sorry if this sounds harsh... Mostly likely a cancer that will kill her if not removed, only a vet can tell you for sure. Could be an abscess, though, which can be sorted with antibiotics. Or even a benign lump that grows slowly & she'll die of something else instead.
I've had horrendous experiences of having lumps removed off of rats, I'd probably not have it done again. My reading of the Fancyrats website and my own experience suggests that only about 25% of lumps are successfully removed (my idea of success, anyway).
How old is your rat, and is her health otherwise good?
Only this morning found a hard lump on one of our fancy mice -- I'm grateful it's a mouse because it's almost sure to be cancer and mice are too small to have lumps removed safely (by and large), so the decision is easy to make.
It sounds like a mammary tumour. This is likely to need removed by the vet. Take her in to see the vet as these are best seen to earlier rather than later. Some can be operated on and some can't. Best to get a vet's opinion though.