I had this same issue, I was down to around 90lbs at 5'5".
MH is right - education is the key. I wanted to fast but was scared I would wither away until I kept reading and understood all I had to loose was the bad stuff. I did the fast and felt great. I did loose some wieght but for about 2 weeks prior to fasting I changed to a completely raw diet, fresh fruits and veggies and then changed to over to fresh homemade juices. I'm doing it again this summer. I would suggest you study up on what the body does when fasting before attempting it. Good luck to you both!!