What Could Be Causing Delayed Period With Cramps After Taking Emergency Contraception?

Good day. I last saw my period on 27th of January then I had unprotected sex, 13th of Feb and later that evening I took Apoil steel pills. My expected next period was 24th of Feb I am having cramps yet no period. I ran pregnancy test on 25th of Feb n it came out negative. What could be wrong with me ?