I’m just going to be blunt: Fasting 16:8 isn’t causing you a stomach ulcer or a flare up of previous stomach ulcers.
If you had never learned that 16:8 is called intermittent fasting, there was always another term for what you’re doing… it’s called skipping breakfast (or lunch or dinner or whatever) and it’s not antagonizing your ulcer(s). Whatever you’re experiencing is either in your head (psychological, related to knowing you’re fasting) or something else is causing the issue. See you Dr. for confirmation.