What is a hard movable lump in lower back that isn't painful, but causes discomfort?

Likely benign. Possible lipoma or fibroma and most likely benign. Can be removed under local anesthesia and should be covered by insurance.
Can be a lipoma or. Most likely -could be a lipoma which is a benign, fatty tumor found in most people & can vary in size and there can be multiple ones. It could also be a cyst of one type or another. Sometimes, after a blunt injury there can be bleeding leading to a contusion or " black & blue" area & occasionally there can be enough bleeding to form a blood collection known as a hematoma that can become hard.