The last rib on the left side is the 12th rib. Hidden pain below the left 12th rib is usually common in left-sided lumbar muscle strain, splenomegaly and left-sided kidney disease. 1. Left lumbar muscle strain: the last rib on the left side is the 12th rib, which is located above the waist, and left lumbar muscle strain can present with diffuse pain on the left side of the lumbar back. 2. Splenomegaly: the spleen is located at the left quarter rib, splenomegaly may present with peri-splenic pain and back discomfort and uncomfortable radiating pain. 3. Kidney disease: the left kidney is located in the area where the 12th rib crosses the spine. Kidney diseases such as left kidney stone, pyelonephritis, and left hydronephrosis can present with dull or aching pain in the lower back. Hidden pain below the last rib on the left side is recommended to go to the hospital for further diagnosis and treatment.