Stomach pain, then vomit. Immediately better. WHAT WAS THAT

My 3 yr old was shouting in pain from his stomach hurting while eating his pancakes this morning. I thought he was hungry or needed to poop. None of that was true. I had even given him metamucil to help with possible constipation. Baby sister ate the same thing and was fine.

I thought maybe it was bad gas pain. So I put him on his tummy and rubbed his back to let out gas (like my mom did with me as a kid). He immediately burped. I was relieved. Then he vomited A TON. After that, he was right back to normal. No fever, no other symptoms. All day he was his normal self.

What do you think that was?!