Hi Dr. I Just Had Hystrescopy Last April Due To

Hi Dr. I just had hystrescopy last April due to cyst and polyps. I was given 3 month cycle duphaston. Is it possible to have a menstruation while taking duphaston? I had my period last May 5 to May 8. Started taking duphaston last ay May 22 but it seems I have a period on the 7th day. Is this normal? (Note: i forgot to take the tablet on the 6th day). Thank you.