You were daughter of Cersei Lannister like King Joffrey, only your Father was not Robert Baratheon, your Father was a worthless blacksmith. So you were a disappointment to the Lannister name from birth.

When your mother abandoned you with the Starks you and Joffrey were only young, and your mother could not explain the situation to him. So she told him you died. But here you are stood infront of him very much alive.

"your...your alive?" he says in disbelief.