I really like this girl, but she has seemed different recently, and I don't know why?

unfortunatley when your in a romantic relationship its hard to be vulnerable ,but you cant avoid that forever.sometimes you have to put yourself on the line for better or for worse.Obviously something is up...and you can try to read her mind and ask all your friends why you think she is being like this and think about it all you want but that would be hopeless. you might never know unless you ask her. try to bring it up in a casual manner. like "hey are you feeling alright lately?" goodluck :)