What Causes Prolonged Brown Spotting With Delayed Periods?

Hello, I have been having a brown discharge for about 7 days now and nromally my period would have came on by now but is hasnt. I dont have any pain, just a dark brown discharge at times. Its not enough to fill a sanitary napkin but I would need to wear a panty liner. Can you tell me what is causing this? Also, Im not on birth control and have not been for about 2 years and no indications of pregancy.