My dog suddenly has no energy?

im gonna take her to the vet tommorow since its closed right now but anyways my dog is a female maltese and quite tiny and usually when i would come see her she would go on her back and let me rub her belly and stuff and usually plays with my other dog but suddenly she has like no energy and her eyes looked very tired and she was acting different and wouldnt stand up and im not sure how old she is but i think maybe around 8 or 9? idk. so any ideas??