Morton's Neuroma: This condition, characterized by a benign neuroma of the intermetatarsal plantar nerve, is a common cause of acute non-traumatic midfoot pain. It typically presents with pain and numbness in the web space between the toes, especially the third and fourth toes.

Midfoot Sprain: Although the question specifies non-traumatic, a midfoot sprain could be considered if the patient experienced a minor, unnoticed injury. It involves ligamentous injury around the midfoot, leading to pain and instability.
Stress Fracture: Stress fractures in the midfoot bones (e.g., navicular, cuboid) can occur without significant trauma, especially in athletes or individuals with increased physical activity. They present with gradual onset of pain that worsens with activity.
Peroneal Tendinopathy: Inflammation or degeneration of the peroneal tendons can cause pain on the lateral side of the midfoot. This condition is common in individuals who participate in sports that involve repetitive ankle motion.
Plantar Fasciitis: While more commonly associated with heel pain, plantar fasciitis can also cause midfoot pain, especially if the fascia is inflamed or irritated in the midfoot region.

Osteomyelitis: Bone infection in the midfoot can present with acute pain and may not always have a clear traumatic cause. It's crucial to consider this diagnosis, especially in patients with risk factors such as diabetes or immunocompromised states.
Deep Vein Thrombosis (DVT): Although less common in the midfoot, DVT can cause acute pain and swelling. It's a critical diagnosis to consider due to the risk of pulmonary embolism.
Compartment Syndrome: This is a medical emergency that can cause severe pain, especially after exercise. It involves increased pressure within a muscle compartment, which can lead to muscle and nerve damage.

Coalition (Tarsal Coalition): An abnormal connection between two or more of the tarsal bones in the foot, which can cause midfoot pain. It's often congenital but may not become symptomatic until adulthood.
Freiberg's Disease: A rare condition characterized by avascular necrosis of the metatarsal head, leading to pain and stiffness in the midfoot.
Kohler's Disease: A rare bone disorder causing pain and swelling in the midfoot due to avascular necrosis of the navicular bone, typically seen in children.