I also have a 17yr old and todays teens are unusual creatures. Talk to her. 99lbs at 5'6 is unusual. My 14 year old at 4'11 with a petite frame is 98lbs. Girls are so worried about fitting in. I would be worried as a mom not to scare you. I would take her to the Dr's just for a check up. You do not have to tell her you are worried yet. It is one of those sticky situations you want them to know and feel that you trust them. Talk to the physician if she is very active in athletics it might be the way she is for her body frame. You will be surprised how many teens have eating disorders and sad to say we as parents tend to get in that denial stage every know and then. I know I do. I have been slapped in the face a couple of time by this. lol
. But I will talk to her and get a doctors advice if you are close to some of her good friends ask them if they feel or think something is wrong. They will tell you if they are a real good friend and they might be worring right along with you but just do not know how to say it. Keep in touch and let us know whats going on.