What Causes Irregular Periods?

i am 24yrs old....had non penetrative sex on 25th june...got pinkish bleeding (first time)on 2nd july followed by light periods(light flow for first time) for 5 days. a lot of cramps n lower abdomen pain since then.negative blood report for pregnancy .ultrasound showed pcos.again had light periods on 25th august....doc suggested meprate .took meprate from 16th sep...still no periods till today(25th sep)