What Causes Brown Discharge And Clots?

Hi i am 29 years old and i am on day 22 of my 28 day cycle. i am trying to concieve my first child. i am not taking any meds or birth control. I have been experiencing brown discharge for 2days. i started to notice it very light on day 21 and now its very thick brown discharge with brown clots. I am about a week before my period is due. any idea what is going on?