The "problem" is that your heart started beating. You've never consciously experienced the feeling of entering sleep and SP, so your body activates the defensive mechanisms. It's like being in a dark-room. You instinctively become more aware, and you sorta go into a defensive position.

It will go away for 2 or 3 tries more, maybe even your next try. Just, relax and ignore it. The most important thing, is to let all go. Focus on how your body feels, without worrying about it though.