I stepped down hard and now my knee is bruised i did not hit it just slammed my leg down

Ligament sprain: You may have a ligament sprain which can cause swelling, bruising and pain apply ice pack, rest for a few days and use a soft knee brace when standing. If there is no improvement see your doctor.

Hard to say without : Knowing your age & physical condition, & not being able to do an exam. If recent injury-try rest, ice pack, possible ace wrap, and elevating your leg. If not improving in next 3-7d, see a doctor to make sure you didn't injure cartilage and/or ligaments.