Can serum electrolytes (such as Ca Mg or K) within NORMAL limits rise or drop substantially within 12-24 hours? Enough to go outside of normal range, even to the point of being symptomatic?
Blood levels for electrolytes are maintained by homeostasis in most people, primarily in the kidneys. But they can actually vary quite widely within hours - depending on health conditions and medications. It can even vary fatally in some (rare) cases.
Why I cannot give a more complete answer:
There are too many possible reasons to discuss adequately in an answer. Therefore it would have to be individualized, which we can't do here.
Gather all your results and take them to a primary care doctor (or GP or whatever they're called where you're from) along with a list of your symptoms (in chronological order) and questions. They can guide your evaluation.
Yes, there are conditions that cause this. One of the most well known is familial periodic paralysis.
Each form of familial periodic paralysis involves a different gene and electrolyte channel. In 70% of affected people, the hypokalemic form is due to a mutation in the alpha-subunit of the voltage-sensitive muscle calcium channel gene on chromosome 1q (HypoPP type I). In some families, the mutation is in the alpha-subunit of the sodium channel gene on chromosome 17 (HypoPP type II).