I have this issue too and mine was gastritis and possibly an ulcer. Intermittent fasting does that to me for some reason. I have been healing it with cabbage juice but I am afraid to fast again due to this. I fast for migraine relief and I’m going to start soon and maybe breaking my fast with the cabbage juice might help.
On my first ~10 fasts I would have stomach pain and feel lightheaded towards the end of my fast if I was doing physical activity during the day. I solved it by listening to my body and eating when my body told me to… not when the clock said “eat”. shortening my fasts was all that helped. I slowly extended my fasts over time and can now comfortably go longer. If you’ve been a snacker for a while your body may just need more time to adjust.
Full disclosure, I am not a daily faster. But, I try to do OMAD or 18:6 a couple times a week.