Why Am I Cramping After My Menses?

I had sex with my bf on the 2nd day of my period which goes for 4 days. I later started cramping a few days after my menstrual periods and i decided to take EC pills (postinor 2) though it was too late for that. I took P2 again a week later after taking the first dose. I have taken 2 pregnancy tests which both turned out -ve but up until now (26 days in cycle) i'm still cramping. I have an irregular cycle that seldom goes up to 28 days. What is wrong with me? I am 25 yr, 5'3'' tall, and weigh 48kg