I notice i feel on/off pressure in left side of stomach, usually when turning body. today, had on/off pressure in left side of stomach when sitting.

Likely gas: What you perceive to be in your stomach is likely behind it in the place in your large intestine where bowel gas often accumulates. It is the highest point in your large bowel when you stand/sit & gas seeks the highest elevation. If you lay down with hips above your shoulders the gas may move towards the outlet so you can pass it.

See your pcp: If the pain is mostly in the upper left of the abdomen, it could be stomach or kidney causes like indigestion, gas, reflux, gastritis, ulcers, and kidney stone.f it is in the left lower abdomen, doctors tend to think of colon conditions (diverticulitis and colitis), gynecologic causes (ovarian cysts or endometriosis), or testicular conditions. i feel you will be best served at ER .if pain worsens.