What Causes Irregular Periods Despite Taking Norethisterone?

I have been prescribed to take Norethisterone 2 tablets twice a day one in morning and evening for 5 days. I have regular period every month my problem started acting up where in it skipping a month then ill get my period the next month for example April - yes, May - no, June - yes, July - no until today, it s almost end of august im still not getting my period. my weight is currently at 90 kilos so I was told that this is maybe the reason why my hormones are acting up. yesterday was my last tablet I want to know when will my period start?