This part has me thinking something has just possessed Spike and the rest of them just don't know it yet...

"Bloody right," Spike agreed. He twitched, his whole body jerking, and two of the potentials just about jumped out of their skins.

"Muscle cramp," Spike said with a grim smile. Xander looked over, and Willow and Buffy and Giles were all looking as confused as he felt. Spike did not do muscle cramps. Vampires in general were not big with muscle cramps."