Bullseye Rash

he acts normal.
bubble eruption disappeared.
the lump has been there for about three weeks now, perhaps with a little whitish red ring around it.

Go to a vet now!
Ring worm is a bacterial infection

I assume that your dog may have picked up some rash on the outside. Honestly, I do not know why the bump may be there, but I want to take your dog to a veterinarian. Sorry I'm not helpful.