it sounds like old blood. sometimes what happens is blood from previous menstruation is unable to pass through the cervix. this is more so in women who have never had children. the blood clots and is broken down. as the cervix opens slightly around menstruation you have brown discharge.
unfortunately this is not something that I can give you total reassurance on as an exam would be very informative as there are many other things that can cause bleeding in the uterus.
I recommend seeing your doctor for an exam just to make sure.
hope this helps,