How bad is the pain if you get amputation from the knee down?

Post-op Pain. It is common to have post operative incisional pain which is worst immediately after surgery and subsides over several weeks. Pain medication helps to manage this. Phantom pain is perceived pain in the amputated limb. People at risk for this had a lot of pain in their limb prior to the amputation. This could last for months/years.
Variable. Depending on your neuropathy preoperatively and how the nerve was handled at amputation the pain can tangle from minimal to severe.