This happened to me also when going back on keto last week. I haven't personally found exactly whats causing it so when it happens i just up the carbs and then try again properly a few days later. I haven't found a pattern to this yet so I just play it by ear.
When the POTS is bad though it makes it very difficult to go through the adaptation process which is difficult enough to begin with.
I've just started again a couple of days ago as my POTS isn't too bad so i'm using this window to try to push through before symptoms flare up again.
i have similar difficulty with keto adaptation. i think there are very few of us who can cycle in and out of keto without some discomfort due to our complex issues. i have tried. i do a higher carb version of keto that keeps me at the low-middle end of blood ketones - along with C8/D-ribose/Inositol/coffee, like
@Learner1 (sometimes i even add a pinch of dextrose to cut the coffee bitterness).
i would also recommend a ton of water and salt during keto adaptation - especially if POTS is acting up.
however, i actually feel it's good for me to carb up occasionally on fruit. i will take a small 1 month break when the crenshaw melons arrive in the farmers market in a few weeks. the foods that really mess me up are carbs like oats and wheat.
i know some of us have commented that keto may not be ideal for all folks with CFS and i would agree because:
1. not all folks with CFS/ME have pronounced SIBO/IBS/carb-intolerance (tho many do)
2. not everyone can tolerate keto adaptation (or complete it)
3. our bodies crave carbs - not naturally an issue because they are supposed to only be periodically available (evolutionarily speaking) - but it is too difficult to get past the cravings for some of us