My dog Penny ( She's a French mastiff ) has been limping for the past couple weeks. I don't know what's wrong with it. :(

It could be a pulled muscle, a slipping joint, a bulging disk in the spine, something in her paw, osteosarcoma... getting the idea? If your dog is having signs of physical problems, it's time for a vet visit.

No way to know for sure without a vet. I agree with froggy!