Growing Pains: This condition is common in children around the age of 5, characterized by recurrent leg pain, often in the evenings or at night, and can alternate between legs. The pain typically resolves by morning. While growing pains are not usually associated with discoloration upon heat exposure, this symptom might be a red herring or related to another benign condition.

Juvenile Idiopathic Arthritis (JIA): This condition can cause joint pain and swelling, which might worsen at night. The pain can move from one joint to another, including the ankles. However, systemic symptoms such as fever, rash, or significant morning stiffness are more typical.
Benign Hypermobility Syndrome: Children with hypermobile joints may experience pain, especially after activity, which could worsen at night. Discoloration with heat could be related to poor circulation or a benign vascular response.
Overuse Injury: Given the age, an overuse injury from running or playing could cause ankle pain that worsens at night. However, this would typically be more consistent in one ankle unless the child has a habit of favoring one leg over the other.

Osteomyelitis or Septic Arthritis: Although less likely, these conditions are medical emergencies. Osteomyelitis (bone infection) and septic arthritis (joint infection) can cause severe pain, especially at night, and can lead to significant morbidity if not promptly treated. Fever, redness, swelling, and warmth over the affected area are typical signs.
Malignancy (e.g., Neuroblastoma, Leukemia): Night pain can be a symptom of bone or soft tissue tumors. While rare, these conditions are critical to diagnose early. Systemic symptoms such as weight loss, fatigue, or unexplained bruising might also be present.
Vasculitis (e.g., Henoch-Schönlein Purpura): This condition can cause joint pain, skin discoloration (purpura), and might worsen with heat exposure due to the vascular nature of the disease.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. While it could explain joint hypermobility and possibly some vascular symptoms, it's less likely to be the primary cause of night pain and heat-induced discoloration.
Fabry Disease: A genetic disorder that can cause pain in the hands and feet, and sometimes skin discoloration, but it's very rare and typically presents with a broader range of systemic symptoms.
Complex Regional Pain Syndrome (CRPS): Characterized by intense burning or aching pain in the hands or feet after an injury or trauma. It's rare in children and would typically follow a traumatic event.