Even with the potion working it's (literal) magic, Merlin still felt a little fragile so his paranoia hadn't had time to mature into full blown activity just yet.
His eyes widened in shock as he heard that but even that hurt so he just rubbed at them instead. He might have thought for an instant that maybe Zach was a) teasing him or b) had actually screwed him but last night was streaming back to him now and he could remember, much like watching a movie of himself, exactly what a jerk he had been. The quiet 'fuck' he muttered was testament enough to that.
He looked up and back at Zach with a wince. "I'm sorry." he apologised whole-heartedly, and although he wondered at Zach's genuinely gentlemanly behaviour he still made as if to go out the balcony door. "I'll let myself out and you'll never hear from me again." he tried to make it sound convicing but it was difficult when he knew it wasn't true. Also... "Apart from something to make up for-" he waved his hand as though he was indicating that he was making up for the majority of Zach's bedroom.