My daughter has been vomiting for about 6 hours?

No other symptoms. She is acting fine. No fever. Just vomiting every 20-30 minutes for the past 5-6 hours (sometimes as often as every 10-15 minutes). I have made sure she has drunk plenty of water to prevent (or try to prevent) dehydration. She is now throwing up clear liquid (presumably the water). Could this just be a small stomach bug or maybe food poisoning?? What else could it possibly be??