Wow, thanks everyone, seems it's more common than I thought - I'd vaguely heard of it before but had no idea what it was.
pm, you seem to have had it really badly. I only get it in my fingers and toes, although having said that I'm the one who pads around our house with 2 fleeces on and a hot water bottle stuffed up them, while everyone else is stifilingly hot.
GP diagnosed it, pm, just by asking a few questions. Have you never seen your GP about it? I suppose if you know what it is and it's not life threatening than there's no need for a diagnosis.
I am, however, a little worried about what the blood tests will reveal. Apart from rheumatoid arthritis, I wonder what else can bring it on. And I wonder if it ever goes away, or at least becomes not quite so severe, over the course of time.
Am off to trawl bd's link - thank you!