Hi Mummytron,
I have had a few similar-sounding episodes over the past few months which I think were exacerbated by the hot weather (in the UK and when were we suffering a heatwave in France). Very unnerving as they come out of the blue and that brings on the immediate anxiety. I decided to get blood tests done for various things (thyroid, iron, diabetes etc) as it sometimes feels like a drop in blood sugar and I wanted to check if it was diabetes. Tests came back ok so I have put it down to hot flushes/hormonal fluctuations. I have eliminated or cut down on some things from my diet (the usual...caffeine etc) and try to keep hydrated and focus on regular healthy nutrition so blood sugar doesn't drop and this has helped.
Hope you don't get many more because they really are awful. X