Bleeding for 3 weeks :(

So i've been bleeding for about 3 weeks now, but its not like a full on period, its just dark brown/black blood and there's not a lot of it either, just spotting really. I'm on the pill microgynon and have been for a year, i've had this spotting before it just appears at random times and i don't have cramps or any other period symptoms so i'm confused, and it's impossible to get hold of my doctor for an appointment any time this week so i was wondering has anyone else experienced this? And do you know what it is? Thanks :)