Can't make it up stairs after walk, walking slower. No power in legs, seems like mostly front legs are weakest. She went down into basement I had to carry her up. She seems to need rest then rebounds. She's eating and drinking water. However this came on all of a sudden where she was actually falling down. (because of front legs). We took Ellie to the vet she had a high heart rate. She's in her seventies. She's fit, what is causing this problem? Thank you!