There could be a few causes for this. The first and most scary is that it's a tumor. It could also be a lipoma (fatty tumor). No matter what, you need to see the vet so that they can aspirate the lump with a fine needle and withdraw some cells. They can then tell you what you are dealing with.
But no matter what, you need to see the vet about this right away.
Let me know if you still have questions.