Fever.

Jen and Lucas were both tipsy when they left the restaurant. Diane had left before them and left the couple alone, but both of them ended up over-indulging themselves so they were driven back to the hotel.

The hotel personnel drove their car to the underground parking, and the couple got into their private elevator. Jen was in a good mood because her mother did not make trouble for her, so she was being very generous and receptive to any affections she received from Lucas.

As expected, that night they ended up having drunk sex with each other before they fell asleep in each other's arms. 

Jen woke up earlier than Lucas on the following day and didn't try to linger in bed like she did the previous day lest they ended up having sex again. She was feeling a bit hungover, so she took some time getting out of bed.