This could have a lot of causes, unfortunately. The most common is a fatty tumor or lipoma. They are very common in aging pets. If this is the case, the lump might move freely under the skin or wiggle under your fingers like a marble.
Aside from that, other tumors, bug bites and even traumas to the skin can cause a lump.
Tumors are not typically painful, if that makes you feel any better.
I would go ahead and see the vet tomorrow for a fine needle aspiration to find out what you're dealing with and then go from there. They can usually do this right in the office.
But my bet is that this is a lipoma.
Let me know if you still have specific questions!