Eating under 500 calories a day?

For the last week, maybe a bit more, I wanted to lose weight. I was eating around 400 - 500 calories a day, skipping breakfast and sometimes lunch. Also, I exercised for about half an hour each day. I realise now that that was a stupid thing to do, however I did lose 4lb in a week. But now that I've stopped I feel really frail and shaky and I've been ill when I've started to eat again. Could I really have done that damage in a week? Or is it all psychosomatic or me being a hypochondriac (which I am a fair bit of the time)?