My dog has been have pretty watery diarrhea. What can i do?

We have taken the 3 year old lad to the vet for a parasite screen. and everything is good. It started about 3 weeks ago when we switched her over to a new food. But that cant still be the problem, can it? I tried pepto bismol and carrots and that didn't help. She acts herself she isn't lethargic and she eats normal and drinks plenty of water. Can anyone tell me what to do?