Lump on Back of Neck. A bump or lump can be classified as any type of enlargement or abnormal bulge under or above the skin. Lumps on the neck are quite common. A lump found on the back of the neck can be formed due to a muscle injury, inflamed tonsils or swelling of the lymph nodes.This can cause a great deal of pain and stiffness in the neck.However, sometimes these lumps can go unnoticed and do not hinder the quality of life of a person.njury of a Muscle or Tendon. A damaged tendon or muscle can be the main culprit behind the swollen bump on the back of the neck. Muscles and tendons keep the body in shape and when one is injured or pulled, the muscle moves from its original position. The damaged part of the muscle sticks out, which forms the lump.