My issue actually started off very much like yours, sudden change in bowels, no BM, even when I finally broke down and tried laxatives, and no relief with enemas. That is when I knew something was wrong.
I found out what was wrong when I took raw garlic and onion several times daily for several days in a row, on the advice of the colonic therapist I was seeing. She also was stunned at the lack of stool after several sessions, as I was eating normally. On the third day of eating garlic, a bundle of worms came out. It was shocking, but atleast I knew.
I had never had a bowel issue in the past, and despite being fatigued and stomach bloated, I looked pretty healthy.
There is a couple of parasiteSupport Forums on this site you can check out to get information and see what others are doing.
My advice, since your story seems very similar to mine in the beginning, would be to start eating garlic several times daily leading up to your colonoscopy. If it is parasites, you will be given a prep for the procedure that should clear stuff out of you. Check it for worms or parasites. It might sound gross, but unless you check with a plastic fork or something, you might never see them. They rarely just float on top.
I would also try more natural herbs to stimulate bowels on their own, like Dr. Christopher's Lower bowel forumula you can get for $10 online many places, or on occasion, Smooth Move tea usually found in the health section at grocery or health food store. The tea should not be used long term, the Dr. Christopher stuff can be used long term if needed.