I would say that this is some type of fungal then. It went from a discoloration a week ago now it looks like this. This could be caused by several things including where you are getting your rats from now. I actually talked about treating sores and stuff in a earlier thread last week a vet check up still may be needed but you can try this for the next 2-3 days and if you don't see a improvement call a vet .
Purchase a pure jar of honey and when I say pure honey it has to be pure. No added sugars ect, straight from the bees into the jar.
Spread it onto a q-tip and apply to the sore 1-2 times a day. Pure honey is a ntaural medicine, you can treat sores, fungals ect. I cured my beardies mild case of mouth rot with it and he had a fungal sore on the outside just like your snake and it cured that to.