Brown discharge instead of period??

Need advice! A little over a week ago, I started getting brown discharge that lasted for about 4 days. I assumed it was just my period starting since it was towards the end of the cycle. However, the brown discharge has stopped and I still have not gotten my period. I took 2 pregnancy tests and they both came back negative. What is the discharge? A period? Spotting? I've never had this happen before. Is it possible to get brown discharge instead of a period? What does that mean?