What do you mean by 'runs'? Is it ALL liquid? Some solids? Watery solids?

It could be the new diet of veggies or it could be some sort of intestinal parasite.

Does the animal appear dehydrated? Slightly pinch(not hard) its skin into a wrinkle along its back, if the skin goes back to its orginal position it should be hydrated. If the wrinkle of skin just remains the same way, I would suggest giving a good soaking for hydration.

If you are concerned, I would call around to a few vets and see if they can do a fecal exam for you. The next time it goes 'poop'......scoop it up with a spoon into a ziplock bag and have the vet check it out