Is this an allergy

[nicnak01]

For the last nine months once or twice a month usually just before bed my son aged 9 will throw up not loads but a mouthful. At first I thought it was when he ate too much but it seems to happen when he has pizza (usually homemade with tomatoes puree but also pizza express and once when he had school dinner he had pizza and come in and threw up when he got in from school) however there are plenty
of times when he has pizza and nothing happens. He also loves tomato soup and is not sick
I always thought allergies where more rashes and diarrhea so is this an allergy and if so what do you do? how do you actually find out what it is making him ill.

[May09Bump]

It can be an allergy or intolerance - vomiting is a natural to get rid of something not right, you don't always get hives. Ask your GP for allergy consultant referral. Not sure how they will test, maybe skin prick or blood test. They could also do a controlled observation where DS would eat suspected food and wait for a response.

Make sure you list what was in the homemade pizza and take ingredient packaging with you to narrow down allergens.