R.R.
Here's some info on IBS ~
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001292/
I have it, along with fibromyalgia. It's no fun and the two worst symptoms I experience are the alternating diarrhea and constipation.
My sister has diverticulitis, which is also an auto-immune disorder like fibromyalgia, and experiences pain much like you describe. Here's some info to check out:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001303/
I would suggest going to the ER the next time you experience the pain, as sometimes things are more obvious when they're happening, (this is how my sister's diverticulitis was diagnosed) and asking to see a gastroenterologist, a physician who specializes in treating the digestive system.
God bless.