Significant bleeding from penis. No pain.

Warning - This is pretty gross stuff.

I woke up this morning to somewhat bloody underwear. A few hours later, my underwear was pretty soaked in blood. When I investigated my underwear further, I saw a very small, membranous looking string of some sort. Very similar to when a woman menstruates.

I'm not experiencing any pain. This is not occurring during urination or ejaculation. I have not experienced an injury of any sort. No rough sex or any trauma. I did not feel or notice the bleeding taking place.

Something similar actually happened to me about 11 years ago when I was 24 (before I had health insurance - a very unhelpful doctor checked my colon, which was fine. no other tests).

I'll talk with my doctor tomorrow, but I'm curious if the community has any ideas. Most of what I see online does not really fit these specific symptoms (usually involves urination, trauma, pain, etc.).

Any ideas?