It is extremely unlikely that the magnesium drink you took made you feel calm, but it is just as unlikely that it made any muscles twitch.
However there is nothing particularly natural in Magnesium citrate, and because Magnesium in a water solution is always an ion, there is nothing special about it. I guess it's not an inorganic molecule like epson salt (magnesium sulphate), and it might be better absorbed than magnesium sulfate.
Since two teaspoons are only 87% of your daily required intake of magnesium, the little bit you took won't have made any difference. Anything you felt it did comes from a placebo effect. Which means you think too much about it. You might want to let your doctor know that you are very amendable to a placebo effect and that a placebo would do wonders for your anxiety.
As long as your kidneys work fine you are unlikely to experience a magnesium overdose, also because if you take too much you'll get diarrhea which will remove the magnesium from your body....