I don't want to alarm you unneccesarily. But if the STRICT rest (preferably cage rest, if possible) and Rimadyl don't work, I would return to the vet and find out if it is possible it could be IVDD (intervertebral disc disease/trauma). How old is she? I have had this problem with more than one of my whippets over the decades. From experience and veterinary advice, I have initially followed the conservative route of a strong dose of Predisone (cortico-steroids) and strict, strict cage rest. (In the case of spinal injuries I have found that Rimadyl doesn't help. If it is a muscle thing, then Rimadyl should do the trick).
On my vet's recommendation, I have reduced the dosage of preds slowly after about 5 days, but still kept the dog as quiet as possible. One can give them Rescue remedy to help with this. If after about 2 weeks there isn't improvement, a scan is usually the recommended thing and then depending on the result - surgery if necessary. I personally would, however, never consider surgery on a dog of more than 10-11 years. Often this type of thing can get better, but the dog may still suffer from the odd episode which can be treated by a couple of days of steroids.
I had one dog who had such a bad neck that he couldn't lift his nose more than about 5 cm from the ground, couldn't shake his head, couldn't turn his head, etc. He was 10 years old. The combination of preds and rest worked wonders, and although he has had one or two minor episodes, he is doing just fine 3 years down the road.
PLEASE keep us posted . . .