If the blood appeared after the biopsies, it is possible that the
small amount of damage done by the biopsies caused some
blood to get into your urine.

By the way, you said there was blood in your underpants, but
didn't say where.  Can you tell for sure that it came from your
penis?  If not, maybe you (or a very, very close friend) can
find some sore, cut, pimple, or whatever that is near where
the blood was found and could be the source.

I'm assuming of course that the blood was in front and not behind,
which could be caused by hemorrhoids or some other rectal or
abdominal problem.