Both of mine were under weight, but were still healthy. So my vet just said to increase food intake.
I have them on ProPlan Senstive stomach canned and dry.
For the past year I would keep feeding them as long as they would eat. Almost free feeding. I use just enough canned food to make them want to eat it.
They NEVER stop running, playing... etc. So like Jada, just can't eat enough food for all the energy they put out!

It's taken about a year and a half, but Buddy's now up to 90 lbs and Sierra is about 65. They are looking MUCH better!

My vet always tells me I'm lucky to have skinny GSD's, but it's also not good when I take them anywhere and people ask
"Are your dogs sick??"
"They're not supposed to look that skinny .. are they??"

I think asking the vet about her weight, and how much you should be feeding her will be the best thing.