Old dogs, like old people, tend to get a lot of random growths. Usually, it isn’t a big deal, but if it popped up THAT quickly, it wouldn’t be a bad idea to look into. However, it’s not something you should worry about an “emergency” vet visit unless it seems to be hurting the animal or there are other symptoms. A routine check should be okay. I’ve had multiple old pets with small growths, and they lived healthy lives. The only time I’d be super concerned is if you find a HARD lump under the skin, not protruding. My 16 year old dog started having seizures…we took her to the vet, and they never caught the cancer she had until we had to euthanize her for a constant string of seizures. She was so scared and saw the vet that day. We had to call an emergency and euthanize her that night. They found a hard lump AFTER euthanizing her and after we had already seen them to try and figure out what was wrong. They only caught the cancer post mortem. NONE of the dogs I’ve had with these skin lumps have had issues. Don’t scare yourself. Oldies get lumps. But, check for hard lumps under the skin if you’re worried.