I had been taking fat 100+ grams for many years, though I increased carbs recent years. Does it fall under pure Keto, I am not sure. My expired strips shows middle of keto spectrum. I am not even sure what the strips showing correct or not. I am more or less intermittent fasting during all this time, though not strictly.It's probably best to go into ketosis before starting a fast, that sure helps. Also having a reason, an aim for why you're doing it also helps a lot. Another thing is that you don't necessarily need to do it if it's causing you problems, best to listen to your body. You can also try intermittent fasting, if you haven't already.
This time, I did water fasting with some Himalayan pink Salt for 5 days. 3rd day is Ok, 4th day starting feeling little issues with lower back, not a big issue, 5th day was rough. I couldn't sit for more than 1 hr ( any where chair or sofa), that forced me laydown all the time. Not a issue, since it is a Saturday. By evening, I started feeling acid reflex. ended up taking digestive enzymes. There is a argument that digestive enzyme needs to digested by stomach. i.e it is fast breaking. Well, I tried with 1 Tbsp Apple cider vinegar with 1 Tsp of lemon juice in a glass of water. It didn't work. some other symptoms are strain sensation in the legs body fat. I did lot of Infra red lamp bath and hot water bath for temporary relief.
I looked on the internet for these symptoms, there are lot of video's talk about these symptoms as a side reaction that can be rough, but goes away in a day or two on its way, if one ride it through. There is no consistency when they will come and go and why. body knows what it is doing.
6th day (Today), I broke the fast with Bone broth( with some apple cider Vinegar) and hoped it will relieve me from all the symptoms, but it took 8 hrs before i can feel little better and took some solids ( eggs + avocado) and I think I am getting back to normal, though the acid reflex is still there. I took digestive enzymes 3 times today. I could sit for a longer time.
I did 17 day Master cleanse ( lemon+ maple syrup+ water ) twice long back( during carb days). I know maple syrup not a ideal one. the experience was not like this.