if you are sure an infection, or virus you may have already had is not the culprit, then it could be a host of other things.
Food allergies you are not aware of, usually you woudn't get"sick" but your face: eyelids, lips, tongue, esphogous, could swell, you could develope a rash on your skin, almost anywhere/develop hives. or run a fever.
Food poisoning, could make you sick, vomitting, diarrhea, fever stomach pains, dizziness, weakness, etc.
Food not properly cooked, and food not properly stored: hot foods really do need to stay hot, and cold foods, cold, when the temperatures change on either end you are enabling bacteria to grow, which commonly causes you to get sick.
Eating expired foods, or foods close to expiration, stale foods, etc. some people tolerate things better than others.
Another thing that can cause you to get sick after eating/or drinking for that matter is water. Nearly every state is different, and especially well water, tastes completely different from tap, and again from bottled water. So, that includes hard water, which is water without its impurities removed. Water that still has that excess of minerals in it that makes it taste coppery, or metalic. So if you can, you can boil the water if you must drink it, and in very pressed condtions, you could drop 15 dropplets of bleech in it to clean it( 15 drops, per gallon of water), its better for an adult to do this if you are unsure how, and to ensure you are not using too much.
Or even some fast foods, yes some people when ordering fast foods/take out, etc. get sick, as in nausea/vomitting, diarrhea, after consumption of fast foods, for a number of reasons, your system does not tolerate how those foods are prepared, or certain oils used in the food, does not agree with you. ie, some people when they eat chinese food, they are fine, but if they eat at a burger place they end up on the toilet.
Or even some pizza places, use more oil, or more salt, on their pizzas than other places, or sauces, and thats one of the reasons the pizza company's food does not taste the same.
just a few pointers, also it always helps to drink alot of water, to help flush, anything out of your system. and if you feel you are not sure, or really sick, see a doctor, or go to the ER, just to be sure things do not get overlooked.