it's just to warn you that you're about to come on. it's basically blood to be honest. blood comes in all sorts of colours and that's just nature's way of preparing you.
Brown vaginal discharge is "old blood." It's caused by several things which are as follows:
Just finished your period & this is left over blood.
Missing of birth control pill.
Break through...
This most likely is a cause of having something, generally a foreign object, pressed into your anal cavity. I would suggest taking a break and seeing if anything improves.