It sounds like you hit your knee pretty hard on that table. Any time you hit any part of your body on a hard object, there is always the potential to break small blood vessels under the skin. This can cause internal bleeding. This small amount of bleeding causes a bruise
. Bruises are typically blue in color and are not a huge concern for your health. Your injury sounds a little different. A reddish bump is usually a sign of a bit more blood and can happen when the injury is more severe. We call a collection of blood like this a hematoma.
Most of the time, these injuries heal themselves. However, sometimes these bruises can represent more severe injuries that should be looked into. If you are a person that has bruised easy with other minor injuries, then you may have a blood clotting disorders
. These are disorders in which the blood does not clot as easily which results in more bleeding when the blood vessels break.
I think that you should schedule an appointment with your primary care physician
. He or she can take a close look at your knee and make sure that the joint is not injured more than a simple bruise.