What Causes Brown Discharge In Periods?

hi, i come on my period 13th of sep and finishedv on the 21st of sep. week or so down the line i had really bad cramp then 2-3 days later when i went to the toilet i had watery blood when i wiped then a couple of days later i had brown stuff i my underwear but it was only a little bit but now its now my underwear its only when i go to the toilet or when i wipe what is it or am i just due on and my cycle is changing??