Two things kind of happened simultaneously. Feeling his tongue brush up against hers, she kind of jumped, mostly because she wasn't expecting it, even if she kind of was expecting it. Well, she knew that kissing involved using your tongue, but feeling it was quite different. And at the same time, she made this little moaning sound in the back of her throat.
Apparently she liked it, since her tongue pressed against his slowly, at first, and then when she got a better hang of it, a bit more urgently. Her nervousness was seeming to fade, at least about kissing him. And she was growing pretty warm, even missing most of her clothes.