A stricture is a narrowing. It would show up on imaging, unless it was at the anus. Anal stricture is a common complication of this surgery. Basically, it is a tightening of the connection between your pouch and anus. Since it is circular, tightening means a smaller opening. If your stool is loose and watery, you may not notice it. But, if it gets too solid or you have too much undigestible fiber, you get blocked.
Sometimes motility of a section of intestine just quits, becomes paralyzed. It could have been that way for a long time, but did not cause problems for you until the stool became too thick.
Since it was the laxative that moved things along, you probably need to really pay attention to your output from now on. This may not be the case for you, but some people find this change to be a permanent problem and have to take gentle laxatives like Miralax on a chronic basis. Be careful with fiber supplements until you know for sure what is going on, as they can create too much bulk.
As always, drink plenty of water and other fluids. I doubt the hard water would be constipating, unless you drink less because you don't like the taste!
Another thought, since you mentioned stress as a factor, is perhaps IPS (irritable pouch syndrome), which is essentially the same as IBS. Usually it is associated with diarrhea, but like IBS, I imagine there could be a constipation type. Hopefully, if this is the case, things will settle down as you settle into your new surroundings.
Jan
