Skin by Donna Jo Napoli (Skyscape hardcover, 6 August 2013).
Sixteen-year-old Giuseppina (Sep) wakes up on the first day of school and is startled when she looks in the mirror to see that her lips are completely white. She can't get an appointment with her doctor until Thursday afternoon, so she has to wait three whole days.
Her mother suggests she wear lipstick until then, which her friends notice, because she never wears makeup. Still it's better than having them see her lips as they really are. She manages to forget about it during the interesting classes like biology, but she runs to the bathroom after class to make sure the lipstick hasn't worn off. In boring English class, all she can think about is her lips.
Then, at lunch, football jock Joshua Winer says he hopes to see her at the party on Saturday, and she's shocked. She was good friends with him back in fifth grade, but since then he became one of the popular crowd and she didn't.
Dr. Ratner doesn't know what could be affecting her, but he takes a bunch of blood tests and suggests a number of possibilities. Although he assures her that whatever it is, it's not fatal, she doesn't like the look in his eyes and begins to imagine all kinds of serious illnesses.She eventually decides to begin a relationship with Joshua before she turns into a freak.
Having gone through something similar when I was sixteen, I empathized with Sep's situation. Her social reactions, such as withdrawing from her friends and isolating herself are credible, as are her emotional ones like depression and irritability.
Sep's acceptance of her illness, which turns out to be vitiligo, happens rather quickly, but that can be attributed to her supportive and understanding family and friends.
Since Napoli usually writes fantasy, I was expecting the cause of Sep's "condition" to be magical, but it wasn't until I finished the book (in one afternoon!) that I recalled this, so obviously it wasn't an issue once I got into the story.
FTC Full Disclosure: I borrowed this book from my local library.