Cold... no. Liv wasn't cold. She was usually more or less room temperature, depending on what the temperature in the room was. She wasn't cold.
She was, however, surprised.
It hadn't been a thought in her head, that her genuine (if slightly more forward) expressions of affection could've been unintentional signals. It was even less a possibility- at least in Liv's mind- that Dean was even remotely interested in her like that, even if they'd joked about it. Now her eyes were wide, her shoulders went straight and he was kissing her...still kissing her...
Kiss him back, genius!
Liv had no idea how long she'd sat there like a mannequin before she actually moved her lips in return, and even then it felt like she failed miserably at a chance she'd probably never get again. But since when did she want a chance to possibly infect her only friend in this weird reality with zombieism?
Since he planted the idea in her head that he might be immune, that's when.