OMG!!!!! Why did my sister all of a sudden CHANGE??? :-O?

I'm 17 and my younger sister is 14. Before school ended my sister wasn't into relationships (dating) or boys. She just was worried about her friends. But recently she's been into guys and she finally got herfirst boyfriend. Shes on her computer from when she wakes up in the morning until she goes to bed and when she leaves her computer shell hide it. She never use to do that before. Also she has a phone and she won't let anyone touch her phone (like my mom)shell flip out and she has never done that before either. She een got worse on her periods. She's such a b..... If you know what I mean. She has been getting in so many fights with my mom and her boyfriend about everything. I'm engaged to a guy but when I was her age I NEVER acted like this and my mom is worried about her. Also shell go to the park and wont tell anyone what she's doing. She isn't the type of girl to have sex. Do you have any advice for me to tell my mom? Or how to handle his situation? Please help!