I noticed that too, and wasn't that the same episode where she wore the scarf around her neck, the same as she did once before, when she was trying to cover up Dracula's bite marks? Maybe she was weak from helping Spike recuperate with a little Slayer medicine, and the explanation for some reason was removed from the final cut.
It would make sense, with Spike being her strongest warrior, that she would want him in top form for fighting, especially since it had already been shown a few weeks earlier in "Sleeper" that he had much more control in sampling her blood than another nameless ensouled vampire had.