What Causes Delay In Periods?

Hi Dr. I used to take contraceptive pills but last month i decided to stop that..and now my periods are 20 days late..i hv checked my pragnency test 3 times all are negative (latest i did today early morning it was negative) i consult my doctor and she suggested me to take Meprate tablet for 5 days today i m copleting the 5 days course but still i m not done with my periods.. please advise i m feeling very bad.

help me please.