Little Things by Rebecca Moesta
The Slayer and her friends discover that saving the world may be one thing, but dealing with life's little everyday gremlins can be quite another! Since her mother's death, Buffy has had a hard time coping. Finding herself suddenly alone and in charge of her younger teenage sister is daunting enough, and being responsible at the same time for saving the world...Buffy is used to dealing with the big stuff, but it's the little things that threaten to get on top of her. She has a toothache; but no dental insurance. Her financial problems are a constant worry -- it's hard to earn a living when you're out every night combating the forces of evil. Nor is she the only one who is having trouble with life's little irritations. Anya and Xander are trapped in their apartment by a plague of ants, and it seems that everyone is trying to put up with some annoyance or other. Then Spike discovers pocket-sized evil wreaking havoc in Weatherly Park, mini-monsters perfectly capable of causing major trouble. In order to stop them, the Slayer's going to have to think small!