Sounds like a coincidence or food poisoning.

Remember real flu (what you get the shot for) is *not* a stomach illness. Actual flu is a very bad respiratory infection. Most of what people call flu (stomach flu) is actually food poisoning or a viral gastroenteritis that makes you throw up, get fever, etc.

Also, some types of food poisoning can happen even if you properly cook the food. Sometimes its not the bacteria themselves that make you sick, but rather the toxins that the bacteria produce and cooking/heat does not always destroy those toxins.