Brown discharge instead of regular period...

F • A almost 17yo horseman. 4h and horses are my life.
During the times when I should be having my period, I get brown discharge instead, like it's almost old blood. Last month I bled for a day, but mostly brown discharge. Recently I have had only brown come put, no red or anything. Any ideas? 17 years old and my periods are not regular. Mom knows about it but isn't worried.