I had a similar experience the first time I supplemented magnesium. Long lasting, non-jittery energy spike. I felt stronger (and empirically could in fact lift more weight), felt better, and was extremely happy. The effect decreased the next few times. After 4 doses (of 50% RDA, spread out over 2 weeks) I began to have adverse effects, including heart palpitation, weakness, and "sense of impending doom".

I wonder if there is a general physiological response to a sudden swing in electrolyte balance that causes the positive effect, rather than the removal of a deficiency.