I'm with Hector - I think she's overdoing the walking on hard surfaces a bit and wearing her pads thin. As she grows, the pads should thicken up and she'll be fine. You might want to stick to dirt and grass paths for a few days though, to give the pads time to recover a bit.
You can also get her some boots if there aren't any good soft places to walk. AllDogBoots (.com) is a good place to get some inexpensive dog shooes. I like the "Meshies" that they sell. They're light weight, inexpensive, and stay on well.