What Causes Scanty Menstrual Bleeding And Brown Vaginal Discharge?

I had a normal periods since I began having them.. At the age of 12 years and 6 months.. I m 21 unmarried , and am a virgin. This year, exactly 4 months ago on 16 June I had my last period.. Then I thought it ll come naturally so I waited for 2 months then there began a pain near kidney area that went down to the abdomen. I had to consult doctor.. The doctors said that s it s urinary infection so they have medicines for that and also medicines to get my period. I waited but I didn t get them.. And the pain on kidney area continued... So I went to a reputed gynaecologist, she suggested for sonography and blood tests.. I got all the reports absolutely normal. And the very next day I got periods ( 19 September 2016 ) and now I feel phobic regarding my periods.. I never had irregular periods and suddenly why this change??? Previously I had normal 5 days.. First 3 days were noraml and 4 day I had lesser and by 5/6 day it was gone... This time I feel it s less... The blood flow is less... I don t know but I m afraid.. Cause now on 4 day I have brown discharge and also on 3 day I didn t have flow... Is all this normal?? My mother says she experienced the same and it s normal since I m getting in my early 20s