I've been on coumadin for almost 5 years now. During this time my periods have been "heavier", but did not last any longer than usual. I agree that you need to talk to your doctor or see a Gnyo.
You need to see a Gynaecologist. He/She will find out the cause. SIX months!? Are you anemic now? Do you top up with iron and vitamin C? Check your Haemoglobin level, please.
One tablet of baby Aspirin can cause my period open up like a fuel pump. My cardio ignored that but the Gynae told me, "DO NOT touch or close by any Aspirin."