Why does left side of my stomach itch inside?

Itching: This is a nonspecific symptom that may simply be due to mild irritability of superficial nerves in the left side of your abdominal wall especially since there is no associated rash or pain. I doubt that it has a serious cause unless you develop widespread itchiness without associated rash. In that case you would be checked for diabetes, hyperthyroidism, electrolyte disorders,etc.