What Causes A Large Clot During Menstruation?

Hi I ve been on clopidogrel since Jan (was on warfrin before they put me on this )as I had suffered a mini stroke in July last year I m type 2 diabetic and also have high bp. In the early hours I came on my period and as I stood up a large blood clot came away it was about 5cm in lenght and about 2cm width. I ve already been on my period 2wks ago and was heavy then, I though maybe it could be to do with not having periods for a while when I had a polip removed in sept 2012 thankyou chimene kirk