My dog isn't being herself?

She is usually very hyper and vocal, but since yesterday she's been lazy and sleeping all day and is just overall not herself. i thought she just might be tired but she's been sleeping all day. she never likes to cuddle and stay still but now if you set her down in one spot she wont move for hours on end. yet she still moves around if you set her on the floor. if you look at her face she looks groggy and sick. i'm taking her to the vet tomorrow but i want to have an idea what's wrong with her