I had gastric flu last November and it knocked me for six. I originally thought it might be food poisoning but it actually came on like flu and I had total exhaustion and didn't move off the sofa for about three days and ate and drank practically nothing for that period. Luckily I started to be able to tolerate liquids after that (recovery drinks were useful). I think I was pretty over the flu itself within a week but it took me longer to recover from the not eating, I felt really weak for a fortnight. A friend of mine was hospitalised because of it as he became dangerously dehydrated so look after yourself.