Furuncle or Abscess: A lump on the lower back, especially when accompanied by a fever that has resolved, could be a furuncle (boil) or an abscess. These are common skin infections that can occur anywhere on the body, including the back. The fever suggests an infectious process, and the spontaneous resolution could indicate the body's response to a localized infection.

Infected Cyst: An infected sebaceous cyst could present similarly, with a lump and associated fever due to the infection. These cysts can become infected, leading to localized symptoms.
Folliculitis: This is an inflammation of the hair follicles, which can be caused by bacterial infection. It could present as a lump, especially if it becomes severe or forms a follicular abscess.
Pilomatricoma: A benign skin tumor that can sometimes become inflamed, presenting as a lump. Though less common, it's a consideration in pediatric patients.

Osteomyelitis: Although less likely, osteomyelitis (an infection of the bone) is a serious condition that could present with a lump and fever. It requires prompt diagnosis and treatment to prevent long-term damage.
Spinal Epidural Abscess: A rare but potentially life-threatening condition where an abscess forms in the epidural space surrounding the spinal cord. Symptoms can include back pain, fever, and neurological deficits. Early diagnosis is crucial.
Tuberculosis: In some regions, tuberculosis (TB) can affect various parts of the body, including the skin and bones. A lump on the back with fever could be a manifestation of TB, especially if the child has been exposed or is from an area with high TB prevalence.

Lymphoma: Though extremely rare in children presenting with a lump on the back, lymphoma (cancer of the immune system) could be considered if other symptoms are present, such as significant weight loss, night sweats, or persistent fatigue.
Neuroblastoma: A type of cancer that can present as a lump in the back, though it's more commonly associated with other symptoms like abdominal masses or neurological deficits.
Hydatid Cyst: Caused by a parasitic infection (Echinococcus), hydatid cysts can form in various parts of the body, including the skin. They are rare in many parts of the world but should be considered in endemic areas or with relevant travel history.