I got a real bad stomach pain after eating peanut , what could be the problem?

Peanut allergy: It is possible that you have a peanut allergy do not eat peanuts or eat peanutbutter until you check with an allergist.

Upset: It may had been coincidental or the peanut may have been spoiled.

Allergy?: You could be allergic to them. See your doctor to get properly evaluated.

Depends: Would have to examine to get a better idea, but one possibility is diverticulitis.