What Causes Brown Vaginal Discharge?

Hello! My period has got me really worried. I am married and not using birth control. We would like to get pregnant. Well, for the last two months I have had brown dischage. It starts out light brown then dark brown. And every two weeks it gets a little heavier but no period. Sometimes after intercourse it is red for a day then back to brown. I have not taken a pregnancy test because i dont feel pregnant. What is wrong with me?