You don't say how old you are....
Brown spotting that lasts and lasts can have many causes, such as too much estrogen, not enough progesterone, other hormonal imbalances, peri-menopause, being very young, weightloss or gain, tumors either cancerous or not, cervical dysplasia, etc.
There really isn't any way you can get a diagnosis over the net. You'd need to go to the doctor as no matter what, you shouldn't continually spot.
I just wanted to try and give you some ideas of things it could be. Try not to worry. It is a VERY common thing with a usually great outcome. Hope you get to the doctor soon and get it taken care of. Good luck to you.