Her mother has just given birth to yet another child. Her Pa wants to start having sex with his wife again, but she pushes him away saying it’s too soon after giving birth.
Pa molests Celie, using her in place of her mother. Pa tells Celie she'd better never tell anybody but God. This may explain why Celie begins each entry by addressing God in little letters/prayers—it’s the only person she has to tell these things to.
Celie’s mom is happier now that Pa isn’t pestering her all the time for some action. But, she’s also sick.