I read a short story MANY years ago (more than 20). A woman saves a small fairy-like creature from her cat. The fairy-creature grants her a wish, and she unknowingly asks to be her cat. Conservation of mass makes her a puma. The spell lasts for a month. She worked in an office-drone type job, with a jerk of a boss. She was about to go on a cruise vacation as when the incident happens (so she's not missed by anyone in her life). She runs across a wildlife biologist type guy, but he thinks she's a wild puma.
She is not in control of the shape-shifting, and it is only temporary. She was described as plump and lacking in self-confidence. If I remember correctly, it was told in first-person, but I might be mistaken.