Not that Lydia was surprised by his strength, because she wasn't, but she was still impressed at how easily he lifted her and carried her to his bed. There was something about being in Derek's bed, though, that made this suddenly a lot more real than it had felt on the couch. Especially when she saw him taking his jeans off.
It was still a little surreal, but the reality of the situation was a bit more heavy when they were in his bed and he was in his boxers, and his hand was up her shirt while they were laying down. She was cold, but not like she had been, and she distracted herself from the residual by pulling away to take off the bulky sweatshirt and then return to him, immediately slipping a hand down below the waistband to curl her fingers around his length. If they were going to do this, she was going to dive right back in, because if she gave herself a second to really, truly process what was going on, she'd back out and things would be immediately awkward.