I Am Having Black Tarry Stools And Also I Blood

I am having black tarry stools and also I blood numbers are all low , three weeks ago I had to 2 pints of plasma and 2 pints I was on warfarin because I had had 2 stents in my in a heart artery. They took me off of warfarin but left me on plavix. Three weeks later i am having the black tarry stools. If it not the blood thinner then what could be causing it?