This happened to me too. If you keep fasting it will work itself out. If you don’t mind it drink pickle juice it helps and it’s natural.https://www.livestrong.com/article/449556-fasting-and-acid-indigestion/
Sort of. I experienced this one time when I started a med that had appetite suppression as a side effect (mine was just weird and presented as upper back pain). Doctors ruled everything else out and said I basically didn’t have enough food in my stomach so the acid being released was coming up. I then took an acid reflux med for awhile until I got my appetite back.
Increase quantity of water to at least 3 L with regular intake throughout the waking day. During the eating period, stay away from curries (duh!). Maybe take a break from IF for a few days and hydrate yourself well and come back rejuvenated.
You may want to cut down the coffee and see if it helps. Drinking black coffee during fasting definitely gave me heartburn. Buy drinking caffeinated tea was okay for me. Don’t go on PPE blockers without doctor’s recommendation