The left side is a common place for problems because that's where the colon switches from transverse to descending colon. Keep taking the P&B and occasionally test like you have been. Enemas, massage, stretching the spine would help greatly too. You hve to get your body to start sending and receiving the correct, healthy messages from the nervous system.
The MP often does mirror the actual shape of the colon! Scary, huh? :)