What Causes Dark Blood In Stool While Had Sex After C-section?

i had my third child by emergency caesarean section five weeks ago. This was my first section. I had sex for the first time tonight and i had no pain but heavy dark blood followed by a large blood clot in the toilet. Why would this happen? i am about 65kg and 5ft 2. My last smear was abnormal, but a colposcopy during early weeks of pregnancy did not show anything of concern. Other than that, my health is good.