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Hello and thank you for using this site to help you. If the dog is unable to use the back legs you need to see your Vet right away or an emergency vet.
Dogs have discs that run through the spine and if these discs rupture the dog looses feeling and use of the back legs.
I would consider this an emergency situation especially since it seems that the dog can do less as time passes. .
Here is more on disc issues , symptoms and treatments
and more here with diagrams
spinal trauma http://www.petplace.com/article/dogs/diseases-conditions-of-dogs/brain-spinal-cord-nerves/spinal-trauma-in-dogs
I see, you did not mention that there was an existing problem,( Hip) just that she was not using her back legs after a day of play and it was getting worse as time passed.
Hopefully the carpofen will alleviate any swelling causing the problem. Keeping her well rested for a few days as well should help. Please let me know how you make out with your vet visit