I see posts like these and I just want to respond to them all but there are so many people here looking ofr help and guidence, and truth is I am too. But DUF- I just wanted to share my experiance, was that I did what Sinead- suggested there, I got the camera up my bum and down my throat too, my poo was tested, my gut was biopsied, I tried several meds, and guess what? The gastro intestinal doc shook his head and not knowing what else to day, said, "Well, theres nothing wrong in there...so my best bet is IBS-C," you can try going to an allergist from here, maybe it's an allergy." so technically I have a clinicle diagnosis of IBS-C but once I came to this site, and started posting my issues, I have had like 4 different people-who know some stuff, say this does not sound like IBS.
What am I to think of that after all these years eh? So like you, I am very anxious about what is really wrong, because it sure is not in my head.
I also experiance the pain of BM, and I don't know if you have looked it up or not, but here is a link you can take a look at, http://www.emedicinehealth.com/rectal_pain/page3_em.htm
the pains may or may not mean something serious, you have to research the symptoms you have with and around it when it happens. I am sorry for you, I know they hurt terribly.
It's not in your head tho', so just put that out of your head ok? Our bodies are very very complex, all you have to do is read a good hand full of these posts from other people, and we are all just as confused and scared as you about what the heck is going on with our bodies, what diet to get on, what foods to cut out, which ones to put in, good fiber bad fiber, good carbs, bad carbs, no carbs, and everyone seems to have a unique little symptom here or there that sets them apart into another category all it's own.
But don't fret over it all too much, go steady on, do read all you can, but try not to think that it all applies either, it can get too confusing. Some people here post some great posts, share with them, you might find someone in nearly he same boat.
Chasing an answer down can be really expensive, and can be really frustrating when you can't find one either, but try not to let it depress you, just get to know your body and what it does, keep a journal, i know it's a pain but it can help a lot. Write everything down.
Best wishes to you,