What's wrong with my cat?

We have three cats. One female and two males, all within the 7–9 month age range. All are up to date on vaccines, but none have been spayed or neutered yet. (Yes, we will have it done within the next two weeks.)

Our female is usually the rambunctious, most energetic of all the cats. Two days ago, her personality completely changed. She isn’t energetic anymore, but she is eating and drinking regularly. The boys have been picking on her, but she hasn’t been fighting back; she usually defends herself pretty well. It’s nothing violent, it’s just the boys’ hormones, which, again, we’ll have taken care of soon. We’re worried because she is usually so boisterous and playful, but now she just lays around and is barely active. We know our cat and we know something is off. Any suggestions? Is this just a phase, because we’d really like our typical cat back? Please help!