What Causes Bruises On Knee?

hi doctor, this morning i woke up and above my right knee i hav 3 bruises kinda stuck together and they dont hurt at all... i dont remember hitting it anywhere or anything.... i'd like to know what its from.... an another thing, on my left leg i had hit it really bad a few months ago and it was bruised pretty bad, the bruise doesn hurt anymore but its not fully heal'd the bruise is still there.... so please if u could tell me i would very much appreciate it