Oh my, I just went through this with my Taz. And by the time I got him to the vet the lump was gone. Although, I can still feel it sometimes. But my vet said it was fat. What? He is 8.5 lbs lighter than he has been at his heaviest.
But I would definitely take him to the vet to have them take a look. Better to be safe than sorry, IMO.

I hope it turns out to be nothing, like Taz's was.