I have three kittens. they are all two months apart starting at 3 months going up to 7 months. I feed them all the same food at the same times. I free feed them dry purina one kitten food and I give them one can of wet food a day. they always have water. but at least on of them, I don't know which, has diarhea. it's very soft, orange, and there's a lot of it. What could the problem be and how do I fix it?