Did you already have GI issues before starting? What sort of diet do you normally follow?
I have an IBS diagnosis, so I already had that problem anyways. It was mostly managed (with carefully eating enough fiber), but then got worse when I started fasting. However, I realized it depended on what I ate to break the fast. Scrambled eggs in butter? Totally fine, shitting like a champ. Homemade keto toast and a cup of coffee? Not at all fine!
Are you aware of some food that is very gentle on your stomach that you can break a fast with? See how it goes? For many people this could be bananas, rice, applesauce, or toast.
This shouldn’t be happening w a 16 hour fast which, btw, you should accidentally done many times in your life.
Try eating small meals
Anyway the more you do it the less it should happen. If not, stop and/or see a doctor.