whats wrong with my mouse?

Ive my two female mice for about 4 months and they were both really lively and great fun but ive started to notice that the most energetic one has started acting weird. She has stopped using her wheel and the tubes, she runs into things and falls off things. she holds her head to the side and when she walks it looks like she is falling sideways. She used to be out all day and all night and only have little naps and now shes asleep all the time. she has been doing this for about a week now. does anybody know whats wrong with her?