Well, im 5'7 too...weight 115 lb, is it count as "Very Skinny"? I seem to know a formula to calculate the body fat, but totally forgot now.
I lost 12 kilos cos I was sick for 8 months. I'm now only 48 kilos, I eat heaps but can't put the weight back on.
I will post a BMI (body mass index) related article in my health blog soon in a day or two, you can check there how much weight you need to be healthy.