I seem to have had an adverse reaction to Vitamin D, although the dosage is within the range that is supposed to be well-tolerated according to studies reported e.g.
here.
I started taking an increased dose in mid-March, totalling about 2250 iu a day, about 1450 of which was D3 and the rest D2.
Either this was too much in itself, or I should have stopped after the spring equinox, when the sun is high enough for it to be possible to make enough Vit D in our skin as long as we get enough sun exposure - and I was getting plenty of that.
I started getting diarrhoea, frequent lower-back pains and polyuria that didn't respond to desmopressin, so was presumably kidney-related and involved excessive excretion of electrolytes in urine (solute diuresis).
Then I had two bouts of spreading unilateral numbness in one day, involving my face and arm, which alarmed me.
Although I don't know the cause of the numbness (maybe electrolyte deficiency? I hope that was all), other symptoms suggested kidney stones, which too much Vitamin D can cause via hypercalcaemia. Whilst kidney stones can resolve, hypercalcaemia can also cause calcium to be laid down in other places, which can be more serious.
I am feeling generally better since cutting Vit D back to about 1250 iu a day, reducing calcium intake (for a while at least), and I have also started taking more magnesium, in the glycinate form.