I have the same issue when I break fasts. I truly believe that it just simply takes more time for the food to make it through your system (since it is empty) and when you go "regularly" usually it is food from 2 or even 3 days ago leaving your body.
Psyllium husk is my go to when I feel like I need a little extra fiber and even in the first few days of my fast (I think of it as a pre-fast) I take psyllium husk and allow myself to drink coffee in the morning.