GliderMommy
Yes, it could definitely be the watermelon. You will see a similar color change if you were to feed them sweet potatoes. Their little digestive tracks react to raw foods in an interesting way. Take him off the 'red' food for a day or so and see if it changes. So long as it is not runny you should be fine. If it starts getting runny, you can offer yogurt - as that seems to solidify it a bit. And last but not least... make sure he stays hydrated. Hope that helps! :)

shadow
yes probably. but if you are really worried it never hurts to go to the vet.