Why is there blood on my dogs butt?

The reason why there is blood on your dogs rear end is becaues he might

have eaten something that is not well for a dog. Or it might be because

he has a kidney infection in witch causes the kidney to draw blood.

If this continues you would need to get to a Vet Technition in wich specializes.

in the central orgin also known as the kidney.

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If is a she,she mite b having her period