What Causes Painful Bruises On Knee, Forearm And Thigh?

I have noticed some unexplained large bruises on my body within the past week. Painful one on my right knee, painful one on my left forearm and now today a very large area on my left thigh that is not painful. I have not bumped into anything lately that would garner such bruises so I'm somewhat concerned. For the last several years I've also had small red dots (like bleeding, can't remember what they're called) all over my arms and upper legs. I've never been diagnosed with a blood disorder or any other serious health issues. And no, I am not taking any medication. Any ideas on what could be wrong?