I finished off a 20 hour fast with some high intensity cardio. After the run, I passed out on my couch for like 30 minutes. After waking up, I had a couple hives on my belly. Maybe I went too hard for my first fast in like months?
I cannot speak to this specific scenario but I have had keto rash before. I never quite figured out which element of keto triggered it in me but it might be a place to start digging. It did eventually go away on its own. Good luck!