Do you have a "dimmer" style rheostat connected to your UTH? If not, just go out and get one. I have used ZooMed UTHs for awhile before and they never gave me a problem, when hooked up to the r-stat.
However, like they said above; it could just be a pre-shed pink belly. Mine all get that.
You may want to switch to a sub like aspen, that forms a thicker "cushion" between the snake and the tank floor. If the snake is on the bare floor after he has moved the towels from over the UTH, that could make a burn occur; esp. if humidity is high and the area over the UTh is wet. But please make sure to use a rheostat!