I absolutely agree. This would cause me a great deal of concern and I would take my dog to the vet. You don't mention breed? If she is a bull breed they are notoriously stoic and will not show signs of pain. There could be something very serious going on here.
It sounds like she has twisted her hip or back doing something. How old is she? Does she have any arthritis?
Also, does she have any hip dysplasia that you know of?
There may be some damage to the nerves going on. If I were you I would take her to be examined by the vet, and in the meantime make absolutely sure she does not go up or down steps, does not jump up onto sofas or beds, or into the car, etc. Keep her walking down to an absolute minimum (just poop/pee walks) and on a leash if you can, to prevent the damage getting any worse.
She may have just sprained something in her hip or back, but it's best to be on the safe side here.