Welcome to the forum.
Many, if not most, crohnies do NOT have a history of digestive problems. There is no known history of digestive problems of any kind in my family - but I most definitely have Crohn's disease. There are many others w/a similar history of no known digstive problems in this forum.
While most crohnies are susceptible to diarrhea, there are quite a few of us prone to constipation instead.
The prednisone reduces inflammation and swelling of the intestines thus allowed things to pass thru that had gotten backed up due to narrowing from the swelling and inflammation. But pred is only a temporary "fix". It is NOT a long term medication, there are too many serious side effects long term.
Most certainly dehydration contributes to constipation. This disease often causes fever, even low grade fevers, due to disease activity and inflammation in the intestines, or wherever, which in turn can lead to dehydration despite no diarrhea.
Ask your questions here. There are plenty of us who have experienced virtually everything you have or will encounter w/this disease and we are willing to share sympathy, what did or didn't work for us, etc. etc.