What Causes A Short Menstrual Cycle?

i am 27 years ihave one child 2 and half years old.i have only three days mensus . but after four days no bleeding.and 9th or 10th day again small bleeding started. that blood colour is dark red .this function continues 5 days .after and after the colour of blood is very light.at last that is white ( this health problem occured after delivery.delivery was siserian)age-27, wheight-46, height-150 cm, early i concelted a doctor.i did scan-sonography report:diaphragm:normal, pleural effusion:nil, liver:normal, gb:normal, porta:normal, spleen:normal, pancreas:normal, right kidney: size 107*46 mm,left kidney:115*51 mm,right ureter and para aortic region and bladder :normal , uterus:size av 80*40*50 mm,myometrium:normal,endometrium:13 mm , vagina,right adnexa,left adnexa:normal remarks:norma