For what symptoms must I seek immediate care after having a below the knee amputation?

Many symptoms. Before or after any surgery, the risks, the potential complications, the potential mortality must be stated to you in a manner that you understand. There should be regular post-op visits to make sure all the healing is going well. Immediate care needs to be sought if there is suddenly new pain or increased pain in the area; or pus, redness, red streaks coming from wound, anything of concern.
Many possibilities. You should contact your doctor following the surgery if you have excessive pain, swelling, or any signs of infection.