Please see my comment on the prior post about diagnosing... but given that you are a mother of twins (at least by name) I would suspect inflammation at the junction between your ribs and sternum, in the area of cartilage that provides for some motion between the two structures. Moving on your side or to your back would pyut different amounts of pressure on this area as the ribcage 'rolls' from the side to flat on the back.
Reflux is a common condition, if the pain is coming more from the middle, right behind the sternum. But a word of caution-- pain and/or pressure in the chest cannot be ruled out for cardiac causes based on exactly where it is felt; the heart causes 'referred pain' to the jaw, shoulder, and arm, and can mimic pain in the gallbladder as well. Although the positional nature of your pain argues against a cardiac cause.
Sounds like the condition I was just diagnosed with, Costochondritis.
Hope this helps.
Mammaoftwins