A small, softish lump (like a small water balloon or similar) is probably not a big deal. They are often lipomas (fatty growths
), fluid-filled cysts, or a sebaceous cyst (sort of like a sweat gland cyst with creamy material inside). All of these are benign.
If the lump feels more hard, I might get a little more "excited", although those can be just firm sebaceous cysts, too.
There are some more concerning type growths, like mast cell tumors
, but they tend to be on top of the skin. Certainly not always, however.
At this point, I think it is reasonable to just monitor the lump and see if it is increasing in size, changing in texture, or bothering your dog. If any of those things occur or the next time you go to the vet for boosters
(whichever comes first), point out the lump to your regular vet. The vet can do a fine-needle aspirate to obtain some cells, which are then examined under the microscope for any signs of malignancy.
But I certainly wouldn't rush to an emergency vet tonight.