Plenty of reasons. Twice i woke up with my pijama pants full of blood, like, full. No scar, no wound, nothing, just blood. I never knew where it came from (as all places from where it could have come were with blood) but I assume it was urinated. It was a side effect of an antidepressant I was taking that start happening two years after beginning the treatment. I had to change the antidepressant and it never happened again