How should i treat severe left calf pain?

Depends on the cause: Calf pain can develop from a partial tear of the calf muscle, a blood clot, a ruptured baker's cyst from behind the knee, contusion/hematoma, strain due to altered gait/limping or it can be referred pain, often from nerve irritation from the lumbar spine. How you treat calf pain differs depending on the diagnosis. Until you see a physician, pain meds, limiting your weight-bearing & icing can help.