Because Cersei must lash out at someone

You feel that you were transported to Westros solely to allow Cersei a way to get her anger out. Your balls have been burnt and crushed to where they are a worthless shriveled bag of dark-blue meat hanging between your legs, the enduring pain making it difficult to walk. After a few weeks of waddling about dodging people in the Red Keep, and cleaning Cersei's arse, you start to walk more normal.

Weeks after Joffrey's funeral, you know is brother is next in line as King. You peek around the corner of the procession to see Cersei dressed in her finest gown beaming with pride at the coronation of her son, Tommen. Perhaps, now she can be happy and give you a break. When the celebration is over, Cersei calls you over and she gives you the look that your services are needed. She doesn't tell you how you are a low life runt while you clean her shitty arse like she usually does, so she must be in a good mood.

As you follow her back to the celebration, Cersei stops and overhears Margaery bragging about Tommen's libido to her handmaidens. You groan silently to yourself. She turns to you and speaks.

"The whore Margery makes me so mad. I'd like to smother her." After a brief pause, she continues, "Come to my chamber right now."

Once in her chambers, she shoves you to the ground and lifts up her long gown. She turns and squats over your face. Her ass lowers and you see a place on the inside of her arse crack where you missed a spot. She settles her weight onto your upturned face. Your nose is wedged into her dirty crack and you have no air to breath. You take it for a while but then you start to squirm. This does not make her lift up. You are in a panic now and are clawling at her hips to let you up. Only then does she give you a command.

"You don't deserve to be here. You can lick my arse. Go ahead. I want to feel your tongue against my arse."

You start to see spots and feel like you are about to suffocate, so you lick and lick. She finally lifts up and stands to lower her gown. You gasps for air over and over.

"I imagined that was Margery and it made me feel so good to make her suffer. I'm going to do that often. And I want all of my handmaidens to take turns with you. Margery is going to lick all of our arses," she said and laughed. Now leave, I must get back to my son, Lord of the Seven Kingdoms Protector of the Realm."