That's troubling to hear. It is a listed risk though with getting a cone biopsy done. I'm not a doctor so I couldn't really tell you what conditions facilitate that type of bleeding, but I'm sure your doctor or the person at the ER who treated you could give you a physiological description of what happened in your body.
However, I did find this article that relates to what happened to you... in case you find it interesting. http://www.greenjournal.org/cgi/cont...act/104/5/1189
One site I read said that heavy bleeding can happen when the scab falls off... and usually it's a delayed thing that happens, just like in your case. That's what I imagine might have happened, although definitely stay in touch with your doctor to inform him/her of any changes. (To make sure you're not getting any infection in there too.)
This website also considers a cone biopsy to be a 'common cause' of heavy bleeding. http://www.hpb.gov.sg/hpb/default.asp?pg_id=865&aid=298