hello . I'm having similar issues myself, although mine stays with me standing up, yours sounds like gastro to me. did they do a upper gi?
I know the uncomfortable feeling you mean, but i've had a chest drain in the area. So it may be nerve related. Straining a chest wall nerve can cause severe pain or discomfort. I've had to have an ER Visit from straining a Left chest wall nerve before, the pain was so severe I thought it was a heart attack.
I think you can relax, pain that is positional in nature is very rarely if ever related to your heart.
Don’t sound at all heart related. Lay on your other side and get some sleep and don’t worry about it.