It's hard to think of possibilities when the two most common sources of your problem have already been looked at and ruled out. The only two other things I can think of would be real long shots. It could be a very odd presentation of a hernia. If hernia were the problem, I would expect it to be worse when erect than when in a horizontal position. If you had diverticulitis in the past, it is possible that some small adhesions formed which are causing some traction on the peritoneum when horizontal. On your next doctor's visit, ask him to examine you for a hernia. I suppose also you could be having some type of intermittent colon spasms, but I can't explain why that would only occur when horizontal. I hope this was helpful.