
What breed is she (or if a mixed breed, what is she most like size-wise).
How much exercise does she have? it could be that she has been doing to much?
Any number of things could be the cause. I would take her to the vets to investigate. Large breed puppies can go through stages where they get stiff, but it's still worth getting them checked out.
Glucosamine is a suppliment for joint care, and is very highly though of by a lot of owners (I've just got my pup on this)