What Causes Delay In Periods?

Doctor I am 38 yr old woman and i have two childs and have a hypothyroid problem but my thyroid is under control and i am taking my tablet but now i have a problem of miss my period my last period was on 29th of Jan and still now i am not getting my period and i am not pregnent and doctor advice me for taking deviry mg three times a day for 5 days so kindly advice me what should i do