Large bruise on back has formed large hard lump and is causing back pain. Bruise is from falling down a flight of stairs (lots of bruises, went to er)?

Collection of blood. The bruising referred to as ecchymosis can become somewhat hardened and circumscribed and referred to as a hematoma and can become quite large. Vascularization in the area will usually resorb the extravasated blood and dissipate, very unusually medical intervention might then be required to remove or drain the site. You can watch the colors change as the blood goes through stages of oxidation.
Hematoma. I would recommend that you should consult a physician. You probably have developed a hematoma which is a collection of blood under the skin. Large hematomas sometimes need to be evacuated surgically. While smaller ones can resolve spontaneously.