Could allergy to watermelon cause blisters on back of tongue?

Enlarged bumps : Very often the bumps or blisters you may be seeing are just enlarges taste buds known as circumvallate papillae . These are the largest papillae on the tongue and at times can become enlarged. Having a dentist evaluate anything that you feel has changed or has recently appeared is always recommended.

Not typical: You could certainly be allergic to watermelon. Typically an allergic reaction would show up as a burning, red and sore tongue. There are naturally large bumps on the back of the tongue called circumvallete papilla. These are normal.