Suggest Treatment For Irregular Periods

Hello,.Im katrina, 20 years old. My problem is that i don't have a regular menstruation and I have not had my period for 3 months and the doctor advised me to take dhuphaston for 10 days . but i skip a one tablet because i forgot to take .is it ok to continue taking it until i reach 10 days or do i need to go back to 1 day until im done taking the 10days of duphaston?and another thing is that ok that im taking vitamins while taking duphaston?