Are you sure they didn't find hemmroids when they did your colonoscopy? I only found out I had some internal ones when my mother, a nurse, looked at my coloscopy pictures; my doctor never told me they were there. In fact, he never called me back to his office to discuss my pictures at all. Mama said a lot of doctors won't talk to you about a test unless there's something wrong with it (which is completely idiotic in my opinion). And apparently having hemmroids is so common they don't consider that anything worth telling you about either.
But you can have false sensations in your rectum without the prescence of hemmroids or any other physical problem. It's just your nerves down there tingling and sending false signals. There are several other people on here who have mentioned sensations similar to yours. One, I think most of us feel at some point in time, is like we're still wet down there, even though the toliet paper comes back clean. Just another type of false signal.