Never, ever be afraid of diarrhea. It is a natural bodily process for quickly eliminating toxic matter from the body and it will end when the toxic level reduces sufficiently. The correct response according to Natural Hygiene is to fast and allow the body to focus on completing this elimination without being sidetracked by processing more food.
As chrisb indicated in a recent post, your body may not be ready to accept food in any form yet and the diarrhea is just what you drank earlier being purged from the body. If this was the case, I would have expected vomiting as the quickest way out but I'm no expert. The other likelihood is that the diarrhea was part of the housecleaning that the body started during your 3 day fast which is now being shifted since you started eating again. Take things gradually and listen to your body for guidance.