My cat has a lump on his neck. He was treated with an antibiotic at the end of July and the lump went away. About a week later it appeared to be back. I called my vet and they put him on a new antibiotic which wasn't as strong as the first. It did not get rid of the lump but the lump did not get any larger. Once we finished that antibiotic it started growing again. It is about the size of a quarter...what should i do?