Ah, I just looked this up a bit better and think I understand a bit more...of course tetany in the usual case is caused by low calcium, and I know I have low blood calcium level from tests. But this hot chocolate, which I haven't drunk since I had these symptoms before (weird, never connected it) also contain high levels of bovine phosphate and are processed with alkali, which, combined with low calcium gives the effect I am getting now. Really horrible! Sorry for such a weird post, I will probably look on this tomorrow in a slightly bemused light but at least my energy is being diverted into my fingers and not pacing around the room...