Unknown allergy!!!!

The first time I noticed a reaction, I had eaten at a mexican restaurant. The reaction started about 1-2 hours after I ate. My stomach started hurting and my body felt really hot. Then my nose was extremely congested and my whole body started itching. I had a rash. It itched even worse on my head. It went away in 30 minutes. 6 months later (now) I ate a hamburger at a nice hamburger restaurant (not fast food). The same thing happened. 1-2 hours after I ate it, my stomach started hurting. I had every symptom that I had the first time, except this time it was 10 times worse. It lasted only 30 mintues. It's very painful, so I'd like to figure out what I'm reacting to so it doesn't happen again.

Thanks!