What Is The Purpose Of Norethisterone?

I have always had regular periods - a long period was 5 /6 days. I started my periods in again in September last year after breastfeeding, they were ok. In January I had a very short break of 23 days and my period then only lasted three days 28 days later in February I stared my period but I am now on day 12 (very light). My doctor has given me norethisterone but I don t know why. Is there a chance it will stop itself and my cycle go back to normal. What does the medicine I ve been given do. Also for your info we have been trying to conceive since January. I had a negative pregnancy test before both periods started in Jan and February.