I've had slight pain/discomfort on my left side below my ribs for nearly one year. Unrelated to food, bowel movements, or position. What can it be?

Be seen. It is likely to be due to your stomach, an ulcer or reflux, but you should be seen to determine if additional testing is needed. If it has been going on that long, you might need a ct scan, endoscopy, ultrasound, or some combination of these.