Hi I am no doctor but in my personal opinion, sound like gas. Do you frequently get heartburn? do you have probs with your bowels? are you over weight?
heart attacks don't have to come on while doing things. Are you feeling anxious, and what about when lying down?
When you say you have chest pains when sitting for a long time, is this because you do not have a good sitting posture and sit slouched?
Do you have any other symptoms with the chest pains, like wheezing, coughing, shortness of breath?
Without any other symptoms, it is likely that it is due to your posture. But to be on the safe side, I suggest you see your doctor to listen to your chest and examine to find out whether this is a muscular, lung or heart problem, indigestion or heartburn.
Best wishes.