What Causes Stomach Cramps After Periods?

Hi i have really bad stomach cramps and when i have noticed i have blood discharge when i go to the toilet, i had my periods on tuesday and they stopped on friday morning, then through out today i am getting really bad stomach cramps and clots of blood but its not red its brown, i recently had unprotected sex with my boyfriend but didnt think anythng of it as i had my period so thought verything was ok