What Causes Inconsistencies In Menstruation?

My period started last November on the 1st. The discharge was slow but I felt no pain whatsoever. But would stop for the next 6 weeks or so. I got worried and went to the doctor on 18 December. I was prescribed Norelut for a course of 20 days and was told that my period would stop and start again after the 20 days. Well, I told one pill in the morning and one at night, never missed any. But the period did not really stop. Traces of blood discharge can be noted especially when I visit the lavatory. Is this normal? Or is something wrong? I am taking my last pill tonight.