I can only guess, I'm afraid.
So don't take this as advice, except that I would definitely see my GP and/or my specialist.
It has the symptoms that something that you have stopped eating has had an effect, possibly some nutrient or say salt is missing from your diet.
I know when I started low carb, that one of the most important change is to take salt, as my previous diet(s), had too much salt and low carb too little. I also had to dehydrate more and drink more water consistently.
It will take time to find the right balance for you.
But keep asking, maybe
@Brunneria, has some insight into muscle problems.
Just a thought, are you doing too much, or too much too soon?
Exercise is important, but because too much can be counterproductive, just take it easy on yourself.