Celie says that her sick, dying mother curses her because
I'm big. I can't move fast enough. By time I git back from the well, the water be warm. By time I git the tray ready the food be cold. By time I git all the children ready for school it be dinner time.
By saying she is "big," Celie means she is pregnant. She is saying that her mother is cursing her because she is pregnant and slow. She can't get water or food to her mother or family fast enough. Her mother claims, with some exaggeration and frustration, that Celie takes all day getting her younger siblings ready for school.
The cursing expresses the mother's distress that the single, teenaged Celie is pregnant. On a deeper level, it expresses the mother's own frustration with her helplessness. She would rather be the one doing the cooking and caring for the children. In her frustration and grief, she lashes out at her daughter.
Although the text doesn't explicitly say so, it is very probable that the mother knows or suspects that her husband is raping and impregnating Celie. Celie says that she felt sorry for her mother and that "trying to believe his [Alphonso's] story kilt her." The last thing her mother needs or wants is Celie as a rival or Celie pregnant and unable to do the work of the house. Therefore, although Celie is a victim, the mother lashes out and blames her for the rape that is going on. Celie seems to intuit that when she tells her mother the babies are dead, her mother strongly suspects this is not so.