Mystery night pain

I wonder if anyone out there can help? I don't know where else to turn after 20 years of trying to understand a seemingly unique condition....
About 20 years ago I started to get a sort of abdominal pain at the weekends when I stayed in bed too long. (I was about 35 then). This pain, a bit like a headache but in my abdomen, would quickly disappear when I got out of bed and moved around. A general aching across the lower abdomen with no single focus of pain.
Over the next 10 years this condition intensified to a state where it would wake me up in the middle of the night, and I would have to get up, walk around for an hour to relieve the pain, and then go back to bed again. Curling up into a ball and bringing my knees up to my chest whilst staying bed helped relieve the pain to some extent. This condition has ended all hope of me ever staying in bed for very long after waking up in the morning!
At no point in my life has this condition ever afflicted me during the day, even when I lie down for a couple of hours sleep in the afternoon...
I have had a variery of doctors performing all manner of tests over the years, up me, down me, acid this, alkili that, sonic scans, MRI scans, X-rays, etc. etc., all to no avail. The only diagnosis anyone could give me was 'IBS', which told me nothing. I have also tried alternative medicines like acupuncture, herbalism, etc.
I have changed my mattress many times, trying different characteristics, all with no real effect. I have tried different diets, seeking to isolate a food which may have had an impact. I have tried various medicines and pills.
The only clue I have is that it might be related to a back operation I had in 1978 (when I was only 27 years old). This operation was successful, and I have had a relatively problem-free back over the years. I am thinking that maybe there is some sort of 'referential' nerve pain from the back to the front, but no-one I have spoke to seems convinced by this, and it does not explain why it only happens at night.
Anyone who has experienced or come across a similar condition, and can shed any light on this, will have my eternal blessings!
Chris

[boron]

Hm.
Pain is in abdominal wall, or inside?
Pain wakes you up, or pain occurs after you wake up and want to stay in bed a little more?

What exactly was back operation, spine? - at which level?