There's a few reasons Elena's having that reaction. She was not expecting to be turned around, or more accurately, was not expecting to be pressed up against Damon so quickly, confidently, and seamlessly. Wasn't expected his lips at her cheek either. Definitely could not have expected that entirely strange contented hum and the nuzzling.
It excites (troubles) her in all bad, bad ways. She doesn't move, doesn't do much at all through her momentary confusion.
All she can think is Damon isn't usually so... close. Comfortable. She rocks with him, into him tentatively, testing (even playful and teasingly), her lashes fluttering. (She doesn't not like this, and maybe her expression isn't as worried as it should be.) But she only lingers for a few beats, her hand reaching to cradle his cheek.
She's going to turn around now, her nose brushing against his. Eye contact would be a bit more preferable. Desired. ]