M.J.
I agree with Kristina. If it's food poisoning, it's a violent reaction. And it's not true that it always happens immediately. For instance, e.coli poisoning can take over a week for symptoms to show up, which is why it's often hard to trace. It varies significantly depending on the type of foodborne illness.
They can evaluate in the ER. I had to go in for stomach flu and I'll spare the details, but I was in horrible shape. By the time they called me in, I was laying on the floor in the ER bathroom in agony (it's that bad that you don't care if you're on the floor in a public restroom). I ultimately needed two bags of IV fluids.
Foodborne illness and stomach flu are very similar. They're both far more horrible than vomiting once. They can determine in the ER which one it is.