What Causes Painful Bruises On The Knee?

Yesterday I went walking with a couple of friends, and my legs ached (like usual) only today I have found bruises all around my knees. One is a very dark red / purple like bruise and they are very sore.
I did not bang my knees or anything yesterday, and I like to think that I do not bruise easily.
Previous doctors have suspected me of possibly having arthritis. I am 20 years old.