Sorry to hear Jenny has a lump. There is no way to know what it is except to have a vet check it.

My Sadie has a fatty tumor at the end of her breast bone that was never even tested because the vet touched it and said it's fine. I of course thought it was cancer but the vet knew instantly it was not. It is squishy but sort of hard too. I can move it around a bit under the skin.

Try to work something out with the vet. To touch a lump should not cost much. I biopsy could get spendy but this shouldn't wait. I know how worried you must be . . . good luck.