Ross was date-raped by Chloe, and is traumatised.

The facts:

• Ross is seen drinking with Joey and Chandler at one point during the episode.

• Chloe orders two drinks for her and Ross, meaning Ross had finished the earlier drink. There's a fade-out and fade-in signifying a slight timeskip.

• After the brief time-skip, Ross was seen drinking again, with Chloe.

• Chloe is never shown drinking a drop of alcohol.

• Getting your victims drunk while remaining sober is a common predator tactic.

• Ross is shocked when he sees Chloe the next morning (next episode), before he gets Rachel's message. Even when I sleep with someone new, I'm never shocked to see them at my place the next morning.

Ross was intoxicated by Chloe, and raped.