Under tank heater and a lamp are both on thermostat?
Bp can bury herself under that substrat directly on the heater and earn a burn, so you have to measure the temps under substrate.
Do you have above terarium lid lamp?
I dont know how your complete set up looks lije, usually we need to know what is the exact set up to come to the possible conclusion.
Ive seen this cases turning out to be a burn or a sensitivity to a substrate or in shedding process some snakes get a pink belly which is normal.

If your set up is fine, than it must be nothing to worry about.



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