Suggest Remedy For Irregular Periods

20years old,weight 160kg unmarried.
Having periods at interval of 30/31days but bleeding continues for 8/9days. I had last period on 14 th Oct. which continued 8/9 days with thick, heavy bleeding. I had sex with my boy friend on 23/24 th day
from starting of period(ejaculation out side). I have missed period-it's 7/8 days over due. Done preganancy test with strips three times -result negative every time. Kindly let me know possible reasons for not having myperiod till now and what medication should I take to have my period.
Thanks