On and off for the last 7 years I have been experiencing what sounds like the same pain as you are having...a dull aching pain in the rectum. When it first started I had the colonoscopy done and was told that I had a few small internal hemmoroids that wouldn't be causing the type of pain I have. I changed my diet and changed medications(the former meds caused constipation) and I am still having the pains. This morning the pain was so intense from the moment I woke and my entire pelvic area was affected and I was doubled up and in tears. I feel that the pain is worse with softer stools and immediately following a bm the spasms start. I'm confused that there seems to be nothing that can be found when I read so many people are experiencing similar pain! My pain comes randomly before, during or after a bm and it doesn't matter if I sit, stand, lie down or walk, when it comes it comes in waves and usually lasts anywhere from a few minutes to several hours. I am a 42 year old woman, with 1 child born c-section but prior to the birth of my child I had a laproscopic procedure done for severe endometriosis. The first time I got the pain it was a couple years after having my child. I read on here that endometriosis was a possible cause for someones rectal pain and I'm curious as to whether my endometriosis could be a possible cause.
It seems you have covered anything I can come up with, other than the fact if you are "Pushing" or forcing a BM, it just could be a weak or sensitive sphincter muscle,especially if you have had a fissure repair, you could try colease, a stool softener to make the BM easier, and make sure you always include fiber in your diet. Good Luck FTB4