It's not confirmed that Kesha has had plastic surgery, but before and after photos certainly make a strong case she had a nose job and maybe even a boob job. Kesha in high school looks literally nothing like she does now. First, she looks kind of tom-boyish and it's like she has no interest in fashion, makeup, or anything girly. Second her nose is bigger indicating that she probably had a nose job.
This before and after picture not only points out the differences in her face, it shows that she may have had breast implants. It's the least likely type plastic surgery she had, but it's still a possibility.
It's different lighting, but it still looks like some plastic surgery altered Kesha's look.