After I eat I get nauseous and sometimes pain in the lower left side of my abdominal. My diet isn't bad. What could this be?

Many things... Many issues from the acid in your stomach to your gall bladder to irritable bowel can easily be a cause. Also other disease processes like Crohn's, ulceratice colitis, and even gluten allergies. More info is needed. You are welcome to make a HealthTap appointment to discuss this further. Hope this helps!