The euphoria is one of the symptoms of Desi becoming a full Slayer.
I don't want to give anything away for readers who live in countries where Season 7 of Buffy the Vampire Slayer hasn't aired yet, but something occurs in the last episode of the show that makes Desi's promotion to full Slayer a possibility.
In our role-playing game, though, Desi doesn't just wake up one day as a full Slayer. She has to buy the Slayer package with experience points. When we played this Episode, she didn't have quite enough experience points to pay for the package. So we instead just foreshadowed the transformation.
Foreshadowing. Ain't we artistic?