Sounds like food poisoning. Probably from contaminated meat.
It is not "the flu". Flu comes with high fever, 22 hours of sleep (cannot stay away), and hurting all over. Gastroenteritis has vomiting and diarrhea, but that is just a general term and not an actual illness. And MOST gastroenteritis comes from eating contaminated food.
Despite the word "poison" it will NOT kill you.
Your body is telling you to NOT put things into your stomach.
Don't eat. Don't drink. Listen to your body.
But you need SOME fluid .. so put from time to time put a small ice chip in your mouth and let it melt. .. but not too much ... too much will only make you vomit again.
After 48 hours, try eating a bit of plain white bread and drinking a bit of water. Try a bit, then wait an hour to see how your body handles it.
Monitor your body for dehydrating: weakness, feeling faint, dry dry mouth. It you become dehydrated, you need to go to the hospital where they will give you fluids through an IV to rehydrate you.
I have food poisoning .. also from a hamburger (not from McDonalds ... my sister and I had gone to a restaurant, both of us had ordered a hamburger .. both of us started getting sick within 15 minutes of each other, and both of us were okay 48 hours later)
After 72 hours, I was fine. Actually by 48 hours, I was pretty good .. eating white bread and drinking water with no problem .. just a bit afraid TO eat too much.
But by 72 hours, I was eating regularly again.
Once you recover, it is good to go talk to the manager at that particular McDonald's and tell them that you got food poisoning from eating one of their hamburgers .. and they need to reinforce the policy of their kitchen help washing their hands.