Peroneal Tendonitis or Strain: This condition is the most likely cause due to the location of the pain (lateral calf) and the action that exacerbates it (dorsiflexion of the ankle). The peroneal tendons run along the lateral aspect of the ankle and are involved in ankle dorsiflexion and eversion, making them prone to injury during such movements.

Lateral Compartment Syndrome: This condition involves increased pressure within the lateral compartment of the leg, which contains muscles and tendons, including the peroneal muscles. It can cause pain in the lateral calf, especially with activities that increase pressure in the compartment, such as dorsiflexion.
Peroneal Nerve Entrapment or Injury: The peroneal nerve wraps around the lateral aspect of the fibula and can be compressed or injured, leading to pain in the lateral calf. Activities that involve dorsiflexion might exacerbate the symptoms if the nerve is irritated.
Muscle Strain (Gastrocnemius or Soleus): While the pain is localized to the lateral calf, strains in the gastrocnemius or soleus muscles, which are involved in ankle movement, could potentially refer pain to this area, especially if the strain involves the lateral heads of these muscles.

Deep Vein Thrombosis (DVT): Although less likely, DVT is a critical diagnosis not to miss due to its potential for severe complications, such as pulmonary embolism. DVT can cause calf pain that worsens with activity, including dorsiflexion of the foot (Homan's sign, though this is not highly sensitive or specific).
Compartment Syndrome (Acute): This is a medical emergency that requires prompt diagnosis and treatment. It can cause severe pain, especially with passive stretching of the muscles within the compartment, and can be exacerbated by activities that increase pressure within the compartment.

Stress Fracture of the Fibula: A stress fracture in the fibula could cause lateral calf pain, especially with weight-bearing activities or movements that stress the bone, such as dorsiflexion.
Popliteal Artery Entrapment Syndrome: This rare condition involves compression of the popliteal artery by the surrounding muscles or tendons, leading to claudication (pain in the leg upon exertion). It's more common in young athletes and could potentially cause pain in the calf with activity.
Tumors (Soft Tissue or Bone): Although very rare, tumors in the soft tissues or bones of the lateral calf could cause pain, especially if they impinge on nerves or interfere with muscle function.