Ruptured Baker cyst: This is the most likely diagnosis given the presentation, as a ruptured Baker cyst typically presents with sudden onset of pain and swelling in the calf, mimicking a deep vein thrombosis (DVT) or other acute conditions.

Deep vein thrombosis (DVT): DVT can present similarly to a ruptured Baker cyst, with pain, swelling, and warmth in the affected leg. It's essential to consider DVT due to its potential for serious complications, such as pulmonary embolism.
Cellulitis: An infection of the skin and subcutaneous tissues, cellulitis can cause redness, swelling, and pain in the affected area, which might be confused with a ruptured Baker cyst.
Muscle strain or tear: Injury to the muscles in the calf can lead to pain and swelling, potentially mimicking the symptoms of a ruptured Baker cyst.

Popliteal artery aneurysm: Although less common, a ruptured popliteal artery aneurysm can present with sudden pain and swelling in the calf and is a medical emergency requiring immediate attention.
Phlebitis or thrombophlebitis: Inflammation of the veins due to a blood clot can cause pain, swelling, and warmth, similar to a ruptured Baker cyst, and requires prompt treatment to prevent complications.

Calf hematoma: Bleeding into the muscle or soft tissue of the calf can cause pain and swelling, but it's less common and might not be the first consideration without a history of trauma.
Soft tissue tumor: Although rare, a soft tissue tumor in the calf could potentially cause swelling and pain, but this would typically be a more gradual onset and might be associated with other systemic symptoms.
Vasculitis: Inflammation of the blood vessels can cause a variety of symptoms, including pain and swelling in the legs, but this is a less common condition and usually presents with other systemic symptoms.