don't want to alarm you but the symptoms you are getting sounds like the build up (which I ignored and put down to IBS stress) to my recent hospital stay - small bowel obstruction and resection. I found I had bowel spasms that came in waves and yes, the pain was massive and I felt sick, and my back & joints hurt. if you aren't farting or having a bowel movement get to the docs (or ambulance in my case) a.s.a.p. I haven't really had problems with diarrhea as such - I get more worried about constipation and trapped wind - the pain is excrutiating and signals a possible blockage. I've had it twice before - due to scarring from my original surgery in 1996 and both times needed treatment in hospital: not necessarily surgery as I was put on a drip for a few days (no food and rested the bowel) and it all cleared up ok.
I'm sure others will add their experiences - just doesn't sound right to me that you're in so much discomfort.
let us know how you get on