What Causes Brown Vaginal Discharge?

Hi. I have been taking the pill Microgynon 30 for a couple of years now. About a week ago i forgot a pill, was under 12 hours. all was fine. I have started to get brown discharge for the past few days, almost a week. I have been abit stressed recently and i think it started round then, im really worried that something is wrong with me or even more worrying that in some magical way im pregnant? Please could you explain this and if i need to do anything then what action is best? thank you