When our dog had arthritis, our vet prescribed Rimadyl, and it worked great, We only gave it to her on bad days, and it definately helped her walk and eased the pain.
This could well be arthritis.
Take her to the vet, because they will do X-rays to see if there is any joint damage, and how extensive it is. Then they will prescribe anti-inflammatories which will help, and reduce any pain she has, and you never know...very soon, she may be running around again with the young 'uns!
Another thing that won't do any harm is to start her on Glucosamine and Chondroitin supplements. This really works, and is completely natural. (if the arthritis is not TOO bad, the Glucosamine/Chondroitin can work wonders)
You could ask your vet's advice about this, and about the correct dosage (I would give her the maximum for her weight.) But ask advice first.