Hopefully its just a fatty lump (Lypoma)... these are quite common in older dogs.
The vet should be able to aspirate the lump and take a look at the cells.
These fatty lumps are usually found just under the skin and are not painful to the dog. They are not attached to the muscle underneath, so you can sort of move it a bit. They are neither hard nor soft, sorta like you'd expect a lump of fat to feel. If this is what you are feeling, odds are this is what it is, but until you know for sure, you can keep up a low level of worrying... sometimes that helps!
Try to get some sleep!
Oh, I forgot to say... Lypomas are normally harmless!