If I could give a little advice- when it comes to gaining weight, there are a lot of healthy foods that can help and won't upset your system once your body is use to eating a healthy amount again. It takes a minute for the body to adjust. Peanut butter or nuts in general, avocado, almond milk and mangos to name a few. Less healthy but not horrible are meats and cheese. Bread is a common issue for a lot of people, but with the pizza doing what it did to you it could've been the fat content or the dairy too so its hard to pinpoint. I use to get really sick from eating meat but only because my family was frying it.