Achilles Tendinitis: This is the most likely diagnosis due to the common occurrence of overuse or strain in the Achilles tendon, leading to sudden heel swelling. Activities that involve running, jumping, or quick changes of direction can cause this condition.

Ankle Sprain: A sudden twisting or rolling of the ankle can cause swelling in the heel area, making it a likely diagnosis.
Plantar Fasciitis: Inflammation of the plantar fascia, a band of tissue that runs along the bottom of the foot, can cause heel pain and swelling.
Heel Fat Pad Contusion: A direct blow to the heel or repetitive stress can cause bruising and swelling of the heel fat pad.
Bursitis: Inflammation of the bursae, fluid-filled sacs that cushion bones, tendons, and muscles near joints, can cause swelling and pain in the heel area.

Deep Vein Thrombosis (DVT): Although less common, DVT can cause sudden swelling in the heel and leg, and is a potentially life-threatening condition if not treated promptly.
Compartment Syndrome: Increased pressure within the muscle compartments of the leg can cause severe pain, swelling, and numbness, requiring emergency medical attention.
Infection (Cellulitis or Abscess): Bacterial or fungal infections can cause sudden swelling, redness, and warmth in the heel area, and may require urgent antibiotic treatment.

Tumor or Cyst: Benign or malignant tumors, such as a ganglion cyst or a soft tissue sarcoma, can cause swelling in the heel area, although this is a rare occurrence.
Rheumatologic Conditions: Certain autoimmune disorders, such as rheumatoid arthritis or psoriatic arthritis, can cause heel swelling, although this is typically accompanied by other systemic symptoms.
Neurogenic Causes: Nerve compression or damage, such as tarsal tunnel syndrome, can cause swelling and pain in the heel area, although this is a less common diagnosis.