If you can see her ribs when she is not moving, or see her spine or hips, she's too skinny. Otherwise she's probably fine, maybe a little underweight.
Feed her a high quality puppy chow, and supplement with eggs, meat, etc. A high quality food is NOT Science Diet, Purina, Beneful, Iams, etc - basically anything you have ever seen a commercial for is crappy food, it means that the cash they should spend on ingredients is spent on advertising, and they add peanut hulls, wheat, corn, or unidentified animal parts to the food, all of which are cheap and unhealthy ingredients that will not provide the nutrition your bitch needs right now.
If you have a Petco, get some Nutro, Blue Buffalo, Wellness, or Solid Gold. Those are all pretty good brands, although the solid gold is probably the best. If you have a feed store near you or a small pet supply store, try to get Innova/California Natural/Orijen/Taste of the Wild or something like that, all of those are higher quality brands.