What Causes Large Bruise On Back Of Leg?

I just noticed a giant bruise in the back of my leg behind my knee. I am taking medication for my heart irregular heartbeats...any connection at all - the dosage is 2 medications per day 25 mg.) and aspirin 80 mg. do I stop the aspirin - do I keep the particular raised? giant bruise: 2 x 2 inches - size of bruise