Tanner and snow both said it looks like a burn. With a burn, your UTH is getting too hot for the snake.

The UTH itself should not be getting more than 92-95ish degrees(this is what mine is at, others may have their UTHs at a different temp). This is so my snakes do not need to burrow to get to a hotter spot.

Can we assume you are not using a thermostat?

What kind of thermometer? A lot of people use and recoomend the accurite digital indoor/outdoor from wlamart. There should be a probe placed over/on the UTH while using a thermostat.