What Causes Menstrual Cycle Imbalance With A Normal Ultrasound?

hello ,last year in november i had some menstrual cycle imbalance and was recommended for an ultrasound....i had also gained weight at that time approx 8 kgs abruptly may be due to change in location or whatever.....after which my gynac suggested modus 10 mg to stop bleeding as nothing came out from ultra sound report......since then i have my periods regular but only for 2 days ...i used to bleed for 4 days before....and its affecting my skin and body weight i feel...can u clarify the actual reason