Mom's please help!

I am a first-time mom and me and my son have been home 3 days now. Earlier today after breast feeding I had the baby's head by my shoulder and was patting his back to burp. After awhile and no burp I went to lay him back in my arms and he was "life-less" meaning his body felt like it had zero weight to it and his arms and head just fell. After moving him around he came back . I was wondering what this could be! ? Could it have anything to do with his neck and him not being able to support it? I'm up worried sick watching him sleep.