If you're not allergic to grapes then eating too many grapes will usually just give you a stomach ache, or diarrhea.
You might also experience Vomiting, indigestion, nausea, dry mouth, sore throat, headaches coughing and sometimes muscular problems too.
Eating too many grapes might also cause you to gain weight due to the natural sugar content and carbs contained in grapes.
If your stomach starts hurting when eating grapes then you've probably just consumed too many grapes in too short of time and should just let your stomach settle before eating anymore grapes.