What Causes Sticky Brown Vaginal Discharge While On Birth Control?

hello there. Just have a question about my period. for the past several months I have just had a very stick-brown clumpy discharge with very little red blood. I am on birth control and I am very active. I ran a marathon in Feb. and I thought I was not having much of a period because I was running so much, but it has been two months since the marathon and i have cut my exercise way down and my period is the same way. I am 23 any ideas?