What Causes Swollen Leg While On Warfarin?

Last night I woke up and felt some discomfort in my left leg calf and when I felt of it, realized it was tender and about a 3" area was swollen - there are some reddish streaks in thd middle of what appears to be a bruise but nothing that looks like a bite. I am on 5.0 mg warfarin with that increased to 7.5 mg on Monday-Wednesday-Friday due to afib - I am a 71 years old caucasian male.

I worked in the yard a lot yesterday so it could be a bruise, but I do not recall anything that I bumped into or injured this particular spot.
Could this be something due to the warfarin?