What is the painful bump on my shoulder blade?

Depends on history - Whether there is an injury to the shoulder blade area or not; in which it could be a fracture or a pulled/torn muscle. Rarely a tumor of the bone or muscle, blood vessel or nerves, or a ganglion cyst, can present a swelling ( a bump). See your doctor to be properly diagnosed.
AC joint. While there are many causes for painful bumps around the shoulder, one of the most common is a painful ac or acromioclavicular joint. It is the joint that connects the clavicle to the scapula and can often become separated or arthritic causing pain.