I Took Norethisterone To Delay My Period On Holiday. My

I took Norethisterone to delay my period on holiday. My period was due on 21st and despite still taking the Norethisterone I had some light bleeding on 28th which then developed into a period as I subsequently discontinued the tablets. Will my first normal period occur on my original due date of 21st and thus compensate for the delay, or do I now have to start counting my cycles from 28th? Just trying to calculate if my fertile days would be the same as we are trying to conceive, or if those will need to be adjusted now. Thanks