What Causes Painful And Sore Arm?

I woke up six nights ago with my left upper arm sore and painful. Since that time, I have had bruising and it is getting worse covering about half of my upper arm. Today, it has been very sore and painful while still continuing to bruise more. I take warfarin and had my protime check three days ago. It was 2.7. What should I do? Do you think that a blood clot is involved?