Oh no! Do you definitely have a stomach flu?
The first time I got sick with a Crohn's flare it was partially brough on by a stomach flu - the only advice I have is to rest and sleep as much as you can - drink as much liquid as your body can handle. Take little sips as often as you can and try and drink water, tea and maybe coconut water if you can get it - I have read that it replenishes the electrolytes in your body better than anything else.
Staying hydrated is the most important thing, hopefully this is just a 24 hour bug that will pass quickly! Sorry you are going through this!