Hello Mrs, sorry to hear about your hair.
It's not happened to me, possibly because I've been losing weight relatively slowly (it's taken me about two months to lose a stone).
I remember reading recently about how it was a common side effect for people doing Lighter Life, which is one of the really low-calorie diets where some people with a very high BMI lose huge amounts like 10 or 11 pounds in the first WEEK!
Having had a bit of look around on the internet, it seems to be common generally for people losing weight rapidly, although I'm not sure if anyone has a definitive reason as to why it should happen.
The bad news is that it seems you can't do much about it, but the good news is that it seems to be temporary so it should come back. Is it really loads you're losing? It sounds miserable.