What Causes Foul Smelling Vaginal Discharge?

hi i had my c-section one year and eight months ago and since that time i never had an intercourse and finally i had one just recently and it was a protected sex and i had this foul smell vaginal discharge after three days so i got so worried. when i was having the intercourse it feels like a virgin again, its very painful and i don't know why. what's the cause of the discharge?