Okay, here is the actual scene in AFFC:

The next she knew, a heavy hand was shaking her awake. It was black as pitch inside the cell, and a huge ugly woman was kneeling over her, a candle in her hand. “Who are you?” the queen demanded. “Are you come to set me free?”    “I am Septa Unella. I am come to hear you tell of all your murders and fornications.”    Cersei knocked her hand aside. “I will have your head. Do not presume to touch me. Get away.”   The woman rose. “Your Grace. I will be back in an hour. Mayhaps by then you will be ready to confess.”    An hour and an hour and an hour. So passed the longest night that Cersei Lannister had ever known, save for the night of Joffrey’s wedding. Her throat was so raw from shouting that she could hardly swallow. The cell turned freezing cold. She had smashed the chamber pot, so she had to squat in a corner to make her water and watch it trickle across the floor. Every time she closed her eyes, Unella was looming over her again, shaking her and asking her if she wanted to confess her sins.    Day brought no relief. Septa Moelle brought her a bowl of some waterly grey gruel as the sun was coming up. Cersei flung it at her head. When they brought a fresh ewer of water, though, she was so thirsty that she had no choice but to drink. When they brought another shift, grey and thin and smelling of mildew, she put it on over her nakedness. And that evening when Moelle appeared again she ate the bread and fish and demanded wine to wash it down. No wine appeared, only Septa Unella, making her hourly visit to ask if the queen was ready to confess. 

(Cersei X, AFFC)

So you’re right, the septas never actually hit her and they gave her food and water in her cell. Not that they are particularly kind, though. Forcing someone to stay awake is a form of torture, especially efficient when it comes to interrogation:

Wake and sleep and wake again, every night was broken into pieces by the rough hands of her tormentors, and every night was colder and crueler than the night before. The hour of the owl, the hour of the wolf, the hour of the nightingale, moonrise and moonset, dusk and dawn, they staggered past like drunkards. What hour was it? What day was it? Where was she? Was this a dream, or had she woken? The little shards of sleep that they allowed her turned into razors, slicing at her wits. Each day found her duller than the day before, exhausted and feverish. She had lost all sense of how long she had been imprisoned in this cell, high up in one of the seven towers of the Great Sept of Baelor. I will grow old and die here, she thought, despairing.

(Cersei I, ADWD)

Cersei’s cell is also “freezing cold“ and all she gets is a shift.

Each night seemed colder than the last. The cell had neither fireplace nor brazier. The only window was too high to allow her a view and too small to squeeze through, but more than large enough to let in the chill. Cersei had torn up the first shift they gave her, demanding the return of her own clothes, but that only left her naked and shivering. When they brought her another shift, she pulled it down over her head and thanked them, choking upon the words.

(Cersei I, ADWD)