GSD way too thin eats well

Question
Sadie is a 3 yr old GSD and is soo very thin ribs are very visable and hr waist is tiny I mean too tiny eats well and all her food seem to act  well not sick normal bowel movements ........enery level high .......help

Answer
It is common for young Shepherds to refuse to eat enough to avoid being on the bony side.  Your description does not match the common problem where my answer is to leave well enough alone.  In general a healthy dog will eat enough to maintain good body condition.  I would do 2 things.  Have the vet check her for tape worm.  Sometimes they are hard to detect.  

How are you determining how much to feed her?  If she will eat it, just feed her more.  The amounts on the packages are only starting points.  With my Labs, when Peggy came to me with her ribs showing, I fed her 7 cups a day of Pro Plan.  To keep Lucky's ribs covered, I fed him about 6 cups.  Most of my similar sized Labs get 3-4 cups to keep them in good body condition.  Unlike most Shepherds, most Labs will eat more than is good for them and look for more.

I would keep offering her more food, until she doesn't eat it all in 15 minutes, and then take it up.  If she hasn't gained weight in a month of that, post back and I will discuss the next step.

Oh, and here is a great illustrated guide to determining your dog's body condition, http://www.puppychow.com/products/popup_body_condition.aspx