MsJames, I know you probably won't find this terribly helpful but I've suffered similar symptoms to what you describe since my teenage years. Thankfully I only get it two or three times a year, but when I feel it coming on, by God, do I pray for death... I'm not kidding, the cramps hurt worse than being in labour. It's horrific.
No doctor has ever offered me a satisfactory explanation for what I go through, and most of them have just scoffed at me (bascially calling me a liar) and refused to even get tests (although tests for what, I have no idea - they seem as baffled as I do). The first time it happened we were travelling (in Townsville on our way to Cairns) and the pain was so bad the nurses had to keep giving me huge doses of morphine to stop me screaming in pain while they tried to xray my abdomen to check my appendix - I came extremely close to them just cutting me open because they thought if it *was* my appendix, they'd best not waste any time diagnosing it. It wasn't my appendix at all, they couldn't figure it out, and the next day the pain had subsided enough for me to go home. Now when I get it, it lasts anywhere between 12 hours and 4 days.

I was lucky to only get the pains once during my pregnancy with DD2, and she was totally fine - at the time I was petrified that I was losing her, but again my doctor wouldn't even send me for a scan to check that she was okay, just told me to let me know if she didn't kick more than once in a 24-hour period. So thankfully your doctor cares enough to refer you for scans to check things out.

I hope it all goes away soon, if it's anything like what I get every now and then I'm really feeling for you right now. I wonder if it might be a stress condition? Either way, it's not pleasant and I hope you feel better soon