I'm underweight and i don't know what's going on with my periods?

i am 5'6 and 109 lbs, i used to be 127 lbs. i was still a healthy weight when i got my regular period june 23-27. i started getting underweight in the beginning/middle of july and i got heavy brown discharge around july 15th. i umed that was my period cause the brown stuff lasted 5 days. i never got any real blood though. now i'm getting that brown discharge again and i'm actually kind of scared about what's going on. could i of lost my period? or is this brown stuff my period coming every 15 days? thank you!