[LunaTheHamster1]

Hi, it can be from food, if you have another grape and wet it and add white bedding or toilet paper to it, you could compare the colours.
He could have just made one heck of a mess with his grape.

There are a couple of other things to try , if you have some hydrogen peroxide you can spray it on the stains and if it fizzes that is a sign of blood. (I don't always find this 100% accurate though as I think it depends on the size of the sample)

There are also human urine strips that test for blood that you can use if there is enough to sample, again not sure of the accuracy. These are just suggestions.

But also needs to be said, as I am not a vet if in any doubt you are doing the right thing in getting a vet appointment. I hope he is ok.