I'm so sorry your little baby has this enlarging lump.. In my experience any lump that changes shape, or size especially if it is growing needs to be checked soon. In the long run by leaving it you could end up with complications that have become a lot more serious & will in the end result in more trauma to your baby as well as bigger vet bills.
Can you take her to the S.P.C.A. I'm not sure what it is called in the states..(Society for the prevention to cruelty to animals) here in SA the underprivileged are helped by their vets but anyone can use them, especially if there is a money problem and your little baby needs help..as it does help with their own running costs. We have friends that only go to them just so they are helping out with costs.
So hope it is not serious & you are able to be helped.
xxAve