Toddler's fracture: A common injury in toddlers, typically occurring when a child falls and twists their leg, causing a fracture of the tibia or fibula. The age and presentation make this a highly plausible diagnosis.

Infection (cellulitis or osteomyelitis): Infections can cause pain and swelling in the affected limb. Given the age of the patient, infections are a common concern and should be considered.
Trauma (contusion or sprain): Accidental injuries are common in toddlers, and a contusion or sprain could cause significant pain.
Growing pains: Although more common in the evening and typically bilateral, growing pains could be considered, especially if the pain is intermittent and not associated with other alarming signs.

Septic arthritis: A serious condition that requires prompt treatment. It can present with pain, swelling, and decreased mobility of the affected joint.
Malignancy (e.g., neuroblastoma or osteosarcoma): Although rare, malignancies can cause bone pain and must be considered to avoid delayed diagnosis.
Non-accidental trauma (child abuse): It's crucial to consider the possibility of child abuse, especially if the history provided does not match the injury or if there are signs of other injuries.

Legg-Calve-Perthes disease: A rare condition characterized by avascular necrosis of the femoral head, more common in children aged 4-8 years but can occur in younger children.
Juvenile idiopathic arthritis: An autoimmune condition that can cause joint pain and swelling. It's less common in this age group but should be considered if other signs of arthritis are present.
Osteogenesis imperfecta: A genetic disorder that affects the bones, causing them to be fragile and prone to fractures. It would be considered if there's a history of frequent fractures with minimal trauma.