Piss is blood, in a sense. It's the bits of your blood that get sieved out and rejected by your kidneys.

Normally those are the only bits supposed to be getting out. But if the filter is busted (kidney trouble) or if the walls of the storage tank it sits in after filtering become damaged (bladder trouble), you can end up pissing actual, unfiltered blood.

Alternatively, you ate something recently with a strong red pigment that can survive digestion, enter the bloodstream, get strained out by the kidneys, and collect in your piss in high enough concentration to turn it red. Beets are a pretty famous culprit.