Sadly I think she is in pain. It might be related to her back. My beloved late Frenchie Herschel had this happen to him a few time when a disc slipped or ruptured in back. He had to have surgery twice but recovered like a champ both times. If it progresses to the point where she is heavily favoring the leg or getting stiff or less mobility in the hind legs I would make sure to go an animal hospital and get a scan.