What Causes Brown Discharge And Spotting During Periods?

ok well i had some blood on the 30 and thought it was my period but it only lasted for a few hours or a day then on the 3rd when i was saposed to have my period i got brown discharge and blood spotting when i wipped i had cramps like if i were on my period but i didnt feel normal i am 17 about 5' and wight about 165 i have no medical history