You could be having menorrhagia
i.e abnormally heavy and prolonged periods
at regular intervals. Is this the first time you have a heavy flow and prolonged period(20 days). How were your previous menstrual cycles? Did it come at regular intervals? Then again quantity of blood flow can be subjective. To get an approximate idea yof blood loss, how many pads to you change per day? There are other causes of menorrhagia like Iron defeciency anaemia
disorders etc. If it is heavy bleeding ,you could even be anaemic (low hemoglobin
hevels). Do you feel tired/ are you looking pale?I would suggest that you visit a gynaecologist for an evaluation.
The colour transition that you mention of brown-red-brown is quite normal.
Have a nice day :)