A hard lump pain on the back waist may be caused by folliculitis, boils, epidermal cysts and other diseases. 1. Folliculitis: Folliculitis may be caused by pathogen infection, skin trauma, excessive sweating and other factors. Patients may have pimples, pustules and other symptoms, and some patients may have a hard lump with pain in the lower back. 2. Boils: Boils are mainly caused by Staphylococcus aureus infection, scratching the skin, sweating and poor hygiene habits. Patients may present with slightly elevated red painful nodules, which may develop into hemispherical firm nodules, manifesting as a hard lump with pain on the hindquarters. 3. Epidermoid cysts: Epidermoid cysts are caused by embryonic ectopic, trauma, surgical operation and other factors that lead to epidermal cell implantation under the skin, local cell proliferation, colloid aggregation. Patients may develop a moderately hard cyst in the region of the hindquarters, accompanied by pain and tenderness. There are many reasons for a hard lump with pain on the posterior waist, and patients are advised to go to regular hospitals to find out the cause of the disease.