Peroneal Tendinopathy or Peroneal Tendonitis: The patient's symptoms of pain along the lateral aspect of the left foot, specifically at the base of the 5th metatarsal, and the aggravation of pain when pulling on the 3rd toe, suggest involvement of the peroneal tendons. The peroneal tendons run along the lateral aspect of the foot and ankle, and their inflammation or tendinopathy could cause the described symptoms, especially in an athlete who recently increased activity.

5th Metatarsal Stress Fracture: Given the patient's recent increase in volleyball activity and the location of the pain at the base of the 5th metatarsal, a stress fracture is a plausible diagnosis. Stress fractures are common in athletes who suddenly increase their training intensity or frequency.
Cuboid Syndrome: This condition involves the cuboid bone in the midfoot and can cause pain on the lateral aspect of the foot, similar to the patient's symptoms. It often occurs after an acute injury or repetitive strain, which could be the case given the patient's volleyball activities.
Lateral Plantar Fasciitis: Although the pain is more localized to the lateral aspect of the foot, plantar fasciitis can sometimes present with pain more laterally, especially if the fascia is irritated near its insertion points.

Osteonecrosis of the 5th Metatarsal: This is a less common condition but could present with similar symptoms. It's crucial to consider, especially if the patient has a history of steroid use or other risk factors, though none were mentioned.
Infection (Osteomyelitis or Soft Tissue Infection): Although the patient denies fever, redness, or significant swelling, an infection could present subtly, especially in the early stages. It's essential to monitor for these conditions, especially if the patient's symptoms worsen or do not improve with rest and conservative management.
Tumor (Benign or Malignant): Though rare, a tumor could cause localized pain and should be considered, especially if the pain persists or worsens over time without a clear traumatic or overuse cause.

Isolated Nerve Entrapment (e.g., Superficial Peroneal Nerve Entrapment): This could cause pain and possibly burning sensations along the lateral aspect of the foot but would more likely be associated with sensory changes, which the patient denies.
Gout or Pseudogout: These conditions can cause acute, severe pain and swelling but typically present with more systemic symptoms and are less likely given the patient's description of mild-to-moderate, throbbing/pulsing pain without significant swelling or redness.