Why My Periods Get Irregular Recently?

My period cycle(exactly 30 days) took ipill after sex(12 hrs after sex) on 8th march. expect date of next period was 1st april but it didnt happen. consulted dr. , she prescribed meprate for 3 days.. no results after 10 days.. again got prescribed for susten300 on 17.. took one pill .. got my period on 18th april.. so next period expected was 18th may.. no period so far.. have had approx 6 home preg tests over the period, all negative. wanna know the reason for irregular or no periods.. havnt happened to me ever dat my periods got delayed..