Food Poisoning, virus, bacteria ...., you'll never know. Just take it easy, drink plenty water and eat lightly if at all. A couple of days of discomfort.
Gastroenteritis can be caused either by a virus (in which case, you'll probably have caught it - either directly or indirectly- from someone else) or by bacteria (in which you've probably acquired it from something that you've eaten):
http://www.nhs.uk/Conditions/Gastroenteritis/Pages/Causes.aspx