Concrete advice: how do I know what I've eaten?
If you ate something "wrong" the day before and feel bad now, but you can not understand what exactly caused your illness, use the old folk method: list mentally or even aloud all the dishes that you have tried. As soon as you get to that "malicious" dish, you will feel literally physical rejection or even nausea.
Such a list is especially effective the next day, when you have already coped with the fire. Your body will already relax, and you will be able to recognize its signals more clearly (in a state of acute malaise, any mention of food can cause nausea and rejection).