Clare,
I agree with the other poster: you really need to start to increase the calories you eat each day. It is extremely dangerous to eat so little.
I am 26 years old, weigh around 135, and I'm about 5'8" tall. This is only two inches taller than you, and I am considered thin for my height. I eat a fair amount during the day, and I can tell you that, from experience, you will actually do better to eat steadily throughout the day rather than starve yourself, as you body will tend to maintain the weight (fat and muscle proportions) if you give it enough to eat in order for it not to go into starvation mode. Remember: your body needs protien to build muscle and, the more muscle you have, the more fat you will lose through exercise and eating balanced meals.
Why have you been eating so few calories??

What do you normally eat on a given day??
I second the recommendation to see a dietician (or at least someone who is very knowledgeable in this area) to guide you to make healthy choices regarding any attempts at weight loss.
~Colleen