If you are using a saddle pad with a synthetic material, could be a
reaction to the heat and synthetic. I've been there. Natural fibers like
wool and cotton are the coolest materials. What worked for me was to
VERY slowly loosen the girth,one hole at a time, thus allowing
circulation to gradually return to the back. When the girth is good and
loose, leave the saddle on for a few moments more.
Sometimes its the friction. Mostly its the heat. Julie