What Causes Light Headed,cramps And Brown Discharge During Periods?

ok im 5ft &1/4in tall, 143 pds and im 20.. every month my periods are very regular, start anywere from the 23rd to the 27 th latest and lasts about 5-6 days, towards the 24th i strtd having a brown discharge, but it wasnt bad only whn i wiped, it lasted prob bout 3 or 4 days and then stopped.. i had cramps and i still get them, i also have been feeling very light headed wen i wake up or stand up to fast.. can u please help me figure this out, i dont have medical to go see a doctor, thnks jess