What Causes Vaginal Bleeding After Sex?

After i had my son i bleed after i have sex just like spotting. i had an abnormal pap when i was pregnant and had that test where they took some chunks out of my cervix. and just the other day it stings when i go pee but i havnt had sex in like a month. i just had my pee sent for sti tests and i came up clean but could it meen i have an sti that u cant detect threw pee?