I hope you've found good answers by now. If not, the only thing I can think of is to not necessarily dismiss the constipation idea since you can poop regularly and still not get it all out. You'd want to ask your doc to order an abdominal x-ray, which would show whether (and how much) your ds was retaining poop. Often the docs will want to feel the belly to see if there's too much poop, but it is NOT accurate.
Were you able to get any answers at today's appt?