My girlfriend suddenly went cold

I (15m) have been with my girlfriend (16f) for almost three years now. We're both pretty strong Christians, who work hard to make each other happy, as well as other people. We went to a youth night, and the guest speaker was talking about pruning, and how it can be used in a spiritual sense. Halfway through the message, (the part talking about letting go of bad branches aka things holding you back) she suddenly went cold. She wouldn't talk to me, and if I said "I love you" she wouldn't return it. I'm at a loss as to what to do. Any advice?