Hi, John, and welcome to the forums!

A little lump could be any number of things, from a harmless birth defect to a tumor or abscess. Does it seem to bother her when you touch it? Can you get in contact with her previous owners and ask if they've noticed it before? If she's had it for a long time and it hasn't caused any problems it's most likely not a problem. On the other hand, a trip to the vet is always the safest course of action.