It looks like a regurg to me too. It should have smelled terrible.

Was this a f/t? How did you thaw it and warm it?

After a regurg you need to give the snake time to build the stomach fluids back up. Give it two weeks before trying to feed again. During this time make sure all your numbers are correct and that the hotspot has not dropped or had something happen to make it not work correctly.

If may be helpful to run us through the type of cage your using with how and with what you are heating with.

I really doubt this was caused by the piece of substrate.