When To Start Taking Duphaston While Having Irregular Periods?

I have been taking Duphaston as a result of PCOS for 4 months. Periods have been irregular, with last month being extremely light. Now this month I have not had a period. How do I know when I should take Duphaston again as I am supposed to take it on day 10- day 24 of my cycle, I don t know which day to use as day 1 now? Please help. Advice as to why I have no period would be great