Lisa's right. Try the crate rest first and see what happens. I would even go for a full two weeks of crate rest just to make sure it's completely healed. If she still comes up lame afterwards, please take her in to the vet. I didn't, and it turned out that Kane had a torn Cranial Cruciate ligament (CCL -- or ACL in humans). If left on it's own, the injury will progress to a full tear (if it's a partial) and will lead to arthritis in the joint later on in life. Kane has a predisposition to arthritis in his knee now, but by nipping it in the bud when I did, I prevented the arthritis from being worse and, with the glucosamine and chondroitin supplements he's on now (and for the rest of his life), he should be fine.
Here are my links on his surgery (TPLO) and the difference between it and TTA.
http://www.gopitbull.com/health-nutrition/39445-kane-surgery-update.html
http://www.gopitbull.com/health-nutrition/39146-kane-has-partially-torn-cruciate-ligament.html
I really hope it's just a pulled muscle for your girl. Kane just got home from surgery this afternoon and I can already tell it's going to be a long road ahead to recovery.
eta: The TPLO was about $3100, but it varies depending on where in the country you live (if you live in the US). I've heard prices ranging from bottom of the barrel $1500, all the way up to $8-9000 in the California area. TTA surgery tends to be cheaper, but it was just recently introduced (in 2004), so a lot of the vets that are out there practicing right now don't have any experience with it (in actually doing the surgery, not necessarily their knowledge of the surgery). In the end, I chose the TPLO because that was the surgery Kane's orthopedic specialist had the most experience in.