Talk to Gastroenterologist on Irritating/Uncomfortable Pain In Abdomen Left Side.

I've been facing a pain issue for 4 months. The pain is bearable and often I ignore it however it makes me uncomfortable or feel it while sitting on floor or surface but when I stands up, straight or lie down on the bed or surface the pain disappears. I've passed through so many tests like Ultrasound, CBC, Urine, Xray, CT Scan. It all seems clear no sign of any infection, no kidney stones, no any other thing. I do not sir on chair for long and I walk too but seems nothing useful. Which doctor is suitable for this issue could you please suggest?