Saying I love you

I told my boyfriend I love him and I meant it. We were close friends for years before we started dating and I really care a lot about him! I don't say it lightly like other couples our age do. But he kind of just nodded mumbled it back, acting disinterested. It really hurt. He normally acts like I'm the most important person in the world -- he's so affectionate and I don't know what to think.