I normally have my period at the start of the month quite heavily for about 5-7 days but this time it only lasted about 4 and was very light. Then after a week I started spotting brown discharge slightly, which then progressed into heavier bleeding (sometime red sometimes brown)The bleeding still hasn't stopped and has been going for 10 days now. I'm worried I'm miscarrying as I had unprotected sex with my boyfriend three weeks ago; or could it be fibroids? And when willl the bleeding stop?

It could be either of those, although bleeding due to miscarriage is usually not that long. It could be due to a hormone imbalance, could be due to a polyp, etc. Since it is not the norm for you, it would be wise to see your doctor on it.