It's a Valentine's Day one. Every Valentine's Day since Xander did the love spell he becomes very romantic towards someone and since he had Anya it always worked out perfect but this year he is alone and living with Spike. Spike ends up phoning Willow because he is worried that something is wrong with Xander because he is being nice to him.
I hope someone can help me! =)
Edit: The story is 'Not Just a Spell' by Nash. Big thanks to themis09