What Causes Prolonged Periods After Having 3 Cycles Of Normal Menstruation?

I noticed that after having about 3-5 normal cycles of menstruation(usually lasting 7-8 days per cycle), there will be a period where i'll menstruate for about 2 to almost 3 weeks(this has happened twice so far: one in april and in september. My periods are never heavy. I'm not sure if there's any problem because when i go to the loo, i'll notice that there's not much discharge on my pad so i'll naturally think that my period is ending but as i clean myself after, there is more discharge. Also i have not been sexually active for the past 2-3 months. My doctor prescribed me norethisterone on both occasions. What does it actually do? I've been reading about it and with all the side effects that comes with it, i'm not too keen on taking it. Whay may be the cause of these long periods? Stress? Or is there like something more serious like a cyst?