Re: What would cause someone to vomit and break out in hives?
A lot of things can cause hives to appear, including stress. And food poisoning, depending on type, can occur up to 48 hours after ingesting the bad food. If it were an allergic food response, you'd expect to find hives in and around her mouth or in the throat and possibly the rectal area after it was passed. It can also cause the BP to fall and pulse to rise.
So chances are it was a stomach virus of some kind and the suddenness of it caused stress hives to occur from the forcefulness of the vomiting.
Hives does not always mean allergy.