I had my period for one day and it was heavy. now for the past 3 days i've been discharging dark brown. what is it and why is this happening?

Can be normal: Or this episode of bleeding might not have been a normal period, but rather breakthrough bleeding from an abnormal cycle. If this happens more, you should be checked out.

Talk to your GYN: You should see a gyn. It could be a miscarriage ( if you are sexually active) or there could be other reasons for a heavy menstrual bleeding.

Brown Discharge: Read this: http://www.Momswhothink.Com/womens-health/brown-vaginal-discharge.Html. As you note, there can be several common causes, ranging from retained uterine fragments to infectious causes. Presumably, there is no chance you could have been pregnant? I would contact your gynecologist and let him know of this "new" feature; he may want you to come in for an evaluation.