It probably has a little to do with hormones but seeing as though she did just have pups but I would say she is reacting this way because the GS thinks that she is the pack (family) leader and that you need protecting whereas it really should be the other way around. You need to learn how to become the pack leader.
Because your other dog is not an "inside" dog like the GS the lab is not part of your pack (family).
I know this sounds kinda silly but dogs are not humans, they don't think the same way people do, underneath their domesticated exterior they really are just like wolves.
What I would suggest is practicing more dominant behaviour over your GS. As soon as you go out in the yard with her poke her in the side (just enough to get her attention of course) and at the same time say NO in a really deep voice. But you have to do this before she reacts to the other dog so she knows you have the situation under control. If she turns away to pay attention to the lab do the same thing until she "submits" i.e. calms down a little.
The GS just needs to be reassured that you are not going to let anything happen to her or the rest of the pack inc. the pups.. I would also suggest not letting the Lab run up the GS at free will, because this can be confrontational which may be why the GS is reacting. Try and practice dominance over the lab as well, if the lab comes running towards you both, even if it is in a playful manner, step in front of the GS and so NO in a really deep voice directed at the Lab and if your dog knows how to sit or lay give the command and tell them to stay.
It will take a few times but eventully she will learn to trust that you will make sure nothing happens to her, her puppies or yourself. She will be a lot less tense and much more relaxed.
You may think this is really crazy, and I thought it was too, until I began dominance training my dog, but you have to put yourself in the same situation.
Let's say your family was a pack of wolves and you just had puppies, if a wolf that wasn't in your pack came running up to you, no matter how friendly they may seem, and the pack leader didn't step in to protect you, you would have to take matters into your own hands right to protect your group right??
I know Dominance training doesn't sound very nice, I mean the the word "dominant" sounds horrible like the act of dominance training would mean punishment or acting really horrible towards your dog but it really isn't. It just means training your dog in a more dog friendly kind of way, a language that they actually understand and respect. I would never hit or abuse my dog in any way and I find this training the best type I have ever used.
I assume that people who use positive reinforcement to train their dogs i.e. using food, think they are being kind too their dogs but feeding your dog overly processed treats constantly is that not cruel??
Anyway - that's kind of off point, you should do whats best for your dog - if you would like to discuss more let me know.
I hope this helps