My Son Just Ate A Piece Of Chocolate With A

My son just ate a piece of chocolate with a peanut. He said his throat started feeling funny. He rinsed his mouth out and I gave him Benadryl. He said his throat feels better, but his stomach doesn't feel good. We have never been told that he has any food allergies, but he doesn't eat nuts often. He is 12. Would his stomach hurt if he ate something he was allergic to?