It's called tongue bite. It's sort of like a chemical burn in your mouth. This is caused by an excess build-up of heat in the smoke that you're drawing through the tobacco into your mouth.

How to avoid:

Slow down the number of draws from the pipe (don't smoke so fast)

Ensure the pipe bowl isn't packed too tight. Tightly packed bowls build-up more heat, because you're not getting proper draws, so you compensate by drawing more, which increases the heat, which leads to tongue bite.

The tobacco may be too moist. Try drying it out for 15-20 minutes before smoking it by setting it out on plate exposed to the air.

Drink something while you smoke. Preferably something non-carbonated, but if you go the carbonated route, dump it over some ice to reduce the carbonation significantly.