I agree- take your snake to the vet. It is hard to diagnose a problem over the internet. And, as Adam said, the misting you do to keep your humidity up may be the reason for scale rot if thats what it is, and it may have caused it the last time your snake had it. And, it could be a burn or a sensitive place on your snake from a new shed. There are too many variables- take your snake to the vet.