How terrible, M.. I feel hot and angry for you because I've had pain that was treated as cavalierly by medical staff. I can hardly believe they gave you benadryl – what was that supposed to accomplish?
The twilight meds don't always work completely. I awoke during abdominal surgery from a twilight sleep; fortunately I had also had a spinal block and so I felt no pain.
But colonoscopies can be extremely painful, like the worst case of gut cramping you've ever had, multiplied and magnified intensely. I know one youngish guy who had a standard procedure without anesthesia, and he said the pain almost did him in. I had a "virtual" colonoscopy several years ago, no anesthesia, and even though no scope was inserted, they still had to inflate my colon with gas and then plug my anus from the inside with a balloon to keep in in. I thought the pain was a lot like unmedicated childbirth, but it took my breath away, or I would have screamed. A lot. This procedure was advertised as being painless, but whoever wrote that copy had apparently never had it done to themselves.
As others have suggested, write down every single detail you can remember. Then talk to your doctor about your experience. He/she needs to know how horrible this was for you. Hopefully, your brutal honesty will spare some other poor soul the same torture. If you don't get an explanation or apology that you find satisfactory, tell them you're going to talk to a lawyer. Then do it. I'm not a person who sues anybody, but I think I would sue your doctor if he was not reasonably contrite.