If you are using her food as training treats through out the day I would measure it all out and put it in a bowl, that way you know how much she is eating.
My 7 month old is underweight, honestly. I have GSD's for almost 30 years and yes she is underweight. My vet alos said free feed her and let her eat what she wants. I did not do that, instead I added 2 more little meals so I know what she is eating. My girls is skinny cause she runs it all off. I am not worried, she will pick up some weight.
If you can see the ribs sticking out from her skin she is too thin. If you feel them and barely see the outline, she is just right.
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