When you first saw Ned bringing a child from the war, you were mad, but really not surprised. You grew up, knowing that men often betrayed their wives, fathering children with other women, mostly whores.
But what really angered you, was the fact that he let his bastard son grow among your own children. Your eldest, Robb and Ned's bastard Jon, became friends. They were brothers after all, but it was uncomfortable for you.
The fact that Ned let his bastard grow in Winterfell, learn the common courtesy and how to fight, even naming him after the man he admired the most - Jon Arryn - lead to conclusion that mother of Jon was not a whore, but someone important to your husband.
You thought it was Ashara Dayne, a woman Eddard loved before you. But when you tried to bring that subject, he quickly and sternly cut off the conversation and told you to never mention Jon's mother again. So you didn't.
And your relationship with your stepson wasn't a great one. He was a walking reminder of your husband's betrayal after all. And when you learned to love your husband, it was even harder for you to even look at Jon.
And Jon, knowing your dislike for him, tried to never get in your way. He was always official with you, never dared to call you a different name than 'Lady Stark' or 'My Lady'. When you were walking beside him, he just bowed his head, but did not speak. He avoided looking at you, as much as possible, and avoided angering you in any way.
A servant walked into your room, to braid your hair for the night, as usual. And when she did so, she used to tell you gossip. And she did it this time as well, telling you about your step son's fight. At first you rolled your eyes, but when you've heard that someone has disrespected you, and he was defending you...
You stood up and without a second thought you left your bedchamber, to meet your husband's bastard. To have a talk with him, for the first time.