Does anyone know???

OK you might have a stricture.  Have you called your surgeon?  Tell his office what is going on.  You could be dehydrated, you could also have a stricture, and malnutrition.  I spent a little time in the hospital myself for these issues...  thew up everything I ate for 2 months...  in the hospital twice and it was a little of everything combined. 

 AND since you are taking in so little, that could be one reason you are constipated.  Little in equals little out.  Plus, the anesthesia takes a huge toll.  Call Dr. H's office for his suggestions.