For md-when lay down w/ back a bit arched & breathe in fully, i feel twinge on right side where the floating ribs are located. also feel gases. cause?

Rib cage: If this position is the only time you're aware of this discomfort, the cause is elusive and it is questionable if studies such as chest X-Ray, rib X-Ray, gall bladder or kidney scans or ultrasounds are worthwhile. I would have a physician examine you first and look for any specific areas of tenderness or look for more descriptive symptoms which might help pinpoint the origin of your discomfort.