Mucus in throat and wanting to vomit. Is the sushi fighting back?

Agree with doc. We find it very difficult to specifically answer these questions without knowing so many other details. By the way, the sushi thing may be coincidental. You might have already been exposed to what's causing your symptoms. This job is a lot about sherlock holmes and listening.
Vomit. There can be a variety of reasons why you have mucus in your throat and feel like vomiting. Certainly, it can be a reaction to something you ate. It can also be due to a variety of other causes. Without more information about other signs and symptoms, it is difficult to pinpoint the problem.