Teenage daughter undressing

Hi everyone, havent been here for a while as things were going as smoothly as possible, but I really need some advice. My step daughter who is 17 has been acting so strange for the last couple of weeks, she has now started to walk around naked and will even start to undress even in front of us all in the living room, we have explained as best we can for her to understand that this is innapropriate, she has been acting strange in school aswell. She says things like everyones going backwards but doesnt know what she means, her tics have increased, its just totally as if she is a different person. It is so awkward as she thinks nothing of doing this infront of her 14yr old brother and her dad. She says she cant walk from one room to another, just stating she cant do it. It is all so odd, we are desperately worried as obvioulsy we know our daughter and none of this is normal for her. She says she knows she is doing weird things but cant give an answer as to why. Please can anyone shed any light and the sudden change, if it was gradual it would be easier to understand but this has happened so suddenly, :(.