Slap on the Head During Meditation

Last night I was meditating, I normally focus on the third eye. I was starting to drift into a dream. Normally when this happens I catch my self within seconds then re-focus my concentration. But for some reason this time I didn't even get into the dream for more then a second. Immediately as I started to drift into it, I felt like I had been slapped behind my head (I was laying down, I even flinched a little). I also heard a popping sound as it happened. It didn't scare me, so I just re-focused on the third eye.

I have a couple of theories.

1. It may have been my spirit guide keeping me awake (I doubt it since I've never consciously made contact with him)

2. Maybe I was about to leave my body, but for whatever reason my sub/conscious "caught" me and I immediately returned.

When I meditate I always make my intention to astral project known, so that's where my second idea came from. I haven't left my body yet, but I should soon. I normally get to the vibration state or sometimes just make a "connection" through the third eye... Not sure how else to describe it. It's normally accompanied with visuals.

I was just wondering if anyone had any ideas of what may of happened

Thanks!