puncture something inside? what is in there, a water balloon? I would see if you can get a different doctor that specializes in Crohn's disease to take a look.
Sometimes, a different perspective can help see the problem for what it is. The problem with a second opinion is this means, having some new stranger poke and prod at your most personal of personal areas. It may be worth the discomfort after dealing with the problem for 18 years?
I myself have lots of rectal pain, that sometimes feels like pressure from the inside-out. Its associated with all the perianal fistulas that will not leave me alone. They come to the surface near the anus. Sometimes, when it hurts really bad, there is nothing I can do. I have to leave work, take a bath, and try to sleep it off. The worst part is, I can't tell anyone around me about it, except for a select few. If I say I don't feel good, they just think I am a whining wimp, making stuff up. One day I am going to drop my pants and show them what I am talking about and let them have nightmares forever!!!!
Whatever you do, keep fighting until you find a solution to the pain. You should not feel guilty for a second about persisting with your doctors until they can do something that will relieve you of the misery. THey get paid pretty well, let them earn that money....
Personally, I think good all Acetimetophen with Hydrocodone (Vicodin, available in all kinds of strengths) is the best pain reliever for that kind of pain. Its addicting though, so limit how many you take.
I myself have to take one most mornings so I can sit down at work without readjusting the pressure on my rear every 15 seconds. It makes it harder to work productively, but I get more done than I would if I was sitting there being miserable all day. I have had the same prescription for 30 a month for about 5 years. As long as I don't increase the amount I take per month, I tell myself I am alright. Even if you get addicted, it can be necessary if you are dealing with managing pain from a chronic condition.