Either not purebred, or very poorly bred. And since you have no idea where the dog originated from ("I recently adopted a German Shepherd who was abandoned at my work") not sure how you concluded purebred.
That size is still within the standard, so not necessarily very poorly bred. I know a very nice female out of very nice lines who is 48 lbs. Both parents were on the low side of standard, but still within it.
That being said, it is hard to tell from the photos if she is a purebred or not. Either way, I wouldn't expect any special health issues.
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