Deep Vein Thrombosis (DVT): This is the most likely diagnosis given the presentation, as DVT is a common condition that matches the symptoms described, such as leg swelling, pain, and warmth.

Cellulitis: An infection of the skin and subcutaneous tissues that can mimic DVT symptoms, including redness, swelling, and warmth of the affected limb.
Muscle Strain or Tear: Can cause pain and swelling in the leg, similar to DVT, especially if the strain or tear is significant.
Baker's Cyst: A fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee, which might be mistaken for a DVT.
Lymphedema: Swelling of the leg due to lymphatic obstruction, which can present similarly to DVT but typically without the acute onset of pain.

Pulmonary Embolism (PE): Although not a direct differential for DVT, PE is a potentially life-threatening complication of DVT. Missing a PE could be fatal.
Phlegmasia Cerulea Dolens: A severe form of DVT that causes significant swelling and cyanosis of the limb, which is a medical emergency.
Compartment Syndrome: A condition resulting from increased pressure within a muscle compartment, which can cause severe pain, swelling, and potentially lead to limb loss if not promptly treated.

Thrombophlebitis: Inflammation of veins caused by a blood clot, which can present with symptoms similar to DVT but is less common.
Popliteal Artery Aneurysm: A rare condition where the artery behind the knee balloons out, which can cause swelling and pain similar to DVT.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling, which might be confused with DVT in its early stages.