What Causes Bruises On Knees And Arms Without Injury?

About three months ago, I woke up one morning with a large bruise on my arm with a knot the center of it. It was a little tender to the touch but didn't cause any severe pain. The bruise lasted about 2 weeks and the knot lasted about a month and a half. Just last week I woke up with the another large bruise with a know in the center on the back of my thigh slighty above the knee area. I am somewhat concerned that this is the second time in 3 months. They were both on my right side. Should I be concerned?