Celie has had two children, but she knows little about her own body. Shug's response to Celie's ignorance is precisely on-target: "Why Miss Celie you still a virgin." That is, Celie has never experienced orgasm or physical pleasure, or even emotional pleasure when she has had sex. Sex, to Shug, is synonymous with delicious pleasure, and if Celie is ignorant of that pleasure, then she is still a virgin to the world of sexuality.
Celie, of course, doesn't know anything about her button of a clitoris and is very naturally confused when she feels shivering, hot pulsing waves of sexual excitement crashing within herself. She is so used to pain that pain seems "normal." She feels guilty about discovering her pleasure center, and it will be a while before she feels free enough to make herself feel good. Shug, remember, is still sleeping with Albert. Celie is sleeping alone. Certainly, Celie could masturbate and bring pleasure to herself, but for the present, she cannot. Celie would like to tell Shug to stop sleeping with Albert, but because she cannot, she masturbates, as Shug has taught her to, but with no pleasure. She cries herself to sleep.
