It was really odd and clued me in a bit but I didn't make the dots until later (Hence I'm a believer of the "Kayla was cheating on Nick" idea, esp with the massage parlor statement too). Either Q was:
1) Talking shit and just making up that he/someone else fucked Kayla. Many such instances of men doing this, not to mention for minority men where "bagging" a white woman (I cannot find a better term for this phrase, sorry!) is like an achievement to boast amongst dudes.
2) He/someone else was with Kayla somehow
3) Trying to get under Nick/Drexel's skin
The way Nick reacted made me feel there was a degree of truth behind the statement under 2) or 3).
If Kayla and Nick had such a good, "aggressively" monogamous relationship, then Nick may have been surprised (and maybe suspicious), but not that disturbed unless he's a paranoid freak (he's not, see the coke stream). To me, it seems Nick was not sufficently entrenched in whatever he beliefs he had about the person involved.