Here is what I have decided.

In the ending scene of last episode alongside seeing Dougie drive, it cuts to Whitney driving for a second. She seems sleepy and her eyes droop for just split second.

I think in the bathroom with the man she did some drugs, presumably coke.

(I had seen a few theories it seemed like they knew each other already. I’ve also seen a few theories that her parents seem to indicate they brought her home from California to keep her out of trouble. If they know each other and she previously had a coke problem, it would make sense for him to assume she would be down for some)

And I think she may be crashing off whatever they did in there, which is why she seems a bit out of it.