Most commonly blisters in the mouth are due to a viral infection. Without a better description I can't really tell you which one I think it is. Several viruses can cause an infection called herpangina that produces small (1-2mm) blisters in the back of the throat and top of the mouth.
The herpes virus (cold sore virus) can also cause very severe blisters in the mouth that often produce blisters on the lips and gums.
It would be uncommon for heat, itself, to cause blisters.