leg stiffness in 8 month old

Question
My 8 month old pup runs, jumps, plays normally.   But I notice after we have been out for our 30-40 minute evening walk she will lie down to rest, and say 20 minutes later when she gets up and starts walking her front legs look really stiff.  But after she stretches or takes 10/15 steps its gone.  Does this sound like a problem?  Or is it just because she hasn't stretched?

Answer
Doesn't sound good.  It is the sort of thing I might say ask the vet about the next visit, but that may not be until she is a year old.  

How is her weight?  Check it, see http://www.puppychow.com/products/popup_body_condition.aspx  Joint problems are much worse in overweight dogs.  

Switch her to adult chow if not already.  It slows growth giving sturdy joints more time to develop.