Weird symptoms -- allergy? IBS?

Sometimes when I eat out (which is not often), about half hour to an hour later, I start getting these symptoms:

-itchy scalp

--feels like it's burning when I scratch, as if I had been scratching til I bleed -- that's how it feels, not that I actually do this

-feels like my ears swell up.

- my face feels swollen, but I get told I look normal

- sometimes my face gets red

And when it's really, really bad:

-nausea

- I feel like im going to pass out while on the toilet (doctors said I dont have colitis)

-not just a red face, but other parts of my body also turns red

-once I got hives

Ive been to the doctors, and they haven't found anything wrong with me. I used to have an intestinal infection last year, but recently have taken blood samples, stool samples, and urine samples -- nothing was found. I have also gotten my heart monitored (my doctor's request) and they said I was fine.

I can't find anything online... one of the doctors said it could be ibs caused by stress?

People with IBS, is this really how it feels like for you as well?

Im thinking about getting tested for allergies. I dont know what else to do since I cant find anything on google that relates :/