Hi OP, it may not be the food it's rejecting, but more what is in the food - normally unhealthy things your body doesn't need! It may have been the bread on the pizza, as if you've stuck to it 100% you haven't had carbs for ages, and your body had a funny turn.
It could be a number of things. If you are worried, you could step up to SS+ and add a small meal a day. Speak to your CDc and see what he/she thinks.
I had a bar for the first time yesterday, and it made me feel sick so it may be the carbs.