I have done 40 day water fast last year in spring and on the 40th day (before I started refeeding) I had to go the hospital because of constipation.
Doesn't make sense.
Why no problems on the previous 39 days??
Were you also dry fasting in part?
If you were drinking water throughout the fast, you should have had nothing in your bowels to get constipated unless you have a blockage.
The only thing other than a blockage that makes sense is if you were also dry fasting during the water fast.
It behooves you to settle this question before another 40 day attempt in my opinion.
If you are having bowel movements the day you start fasting and drink distilled water for the 40 day fast, you shouldn't have any problems.
The reason you fast is to cleanse the body. The cleansing agent is the water. If you don't drink water the cleansing comes to a halt and anything in the bowel will harden.
If there is a reason to dry fast, I never understood why.