I would add too, that if you're having other symptoms, it could be a mild intestinal virus. We certainly aren't immune to those!
If it's a transient problem and doesn't occur very often, I wouldn't worry to much about it. It's common to have loose watery stool if you have skipped meals, or if you've gone a long time between eating. But otherwise, as Scott suggests, it's a good idea to look back on what you've been eating. Blueberries, blackberries, red cabbage, black or purple licorice or wine gums, blue, green or purple gatorade, etc, can all turn your stool green. Iron supplements, if you're taking them, will do the same. If it's something as simple as tracing it back to a certain food, this can give you peace of mind.
FYI - I have purple stool when I eat blueberries or blackberries. I still find it amusing. LOL