What Causes Light And Short Menstrual Cycle?

hello dr,im from pakistan i want to ask a question u kindly mail my answer h im 22year old girl,unmarried wt 45 kg height 5 feet ,i am facing problem in my menstrual cycle since september of this year,my menstrual bleeding occur only 2 days before that it was normal ocur 4 to 5 days now it occur only 2 days or if i take bath during my menstrual cycle or ttook some painkiller for headache bleeding reduces even more like this month i took painkiller on ist day and my menstrual flow stops not occur even on 2nd day i have done ultrasound pelvis but it was normal and my bood cp is also in normal range what should i do why it happens and kindly suggest me treatement