What Causes Missing Periods?

hi doc, i am 22 years old girl, married 1 year ago.i used to get my period in every 10 days.so, due to my irregular period, doc advised me to take yamini tablet.so, i continued to take it for 8 months. but i have stopped it in the 9th month,and i haven't got my period yet.i had taken the last tablet on 20th march and got period from 25th march to 29 march.what would be the problem.docter, please help me. i m very much worried abt it.