What Causes An Early Menstrual Period After Missing Dronis Tablets?

hello, i am 29 yrs old,a mother of 3 years old .In January 2014 i suffered a miscarriage, it was just 5 weeks pregnancy.After some tests my doctor said that i hv minor cyst in my ovaries. She asked me to take DRONIS 30. for the past 5 month i m taking that medicine as prescribed.I got my periods on 8th july and i started taking the medicine same day,but unfortunately i missed it thrice this time..and now my periods have started that is today 20 july. I am really scared,is it normal..if not kindly help!! thanx mrinalini.