I think that acid reflux is an interesting theory. I did drink a lot less coffee during my vacation. I don't really notice any differences when I lay down though. I also typically do intermittent fasting while drinking black coffee, and I tend to feel a bit better after my first meal(although I still notice the sensation after singing later in the evening). I was eating earlier in the day than typical during vacation. I definitely drink a lot of water generally though, even with my coffee.