Any ideas on what just happened?

First off, this isn't spam, I'm fairly new but I think you will be able to tell I'm not trying to start sh*t.
 
Basically, I got stoned off a couple hits with my bong (bottle bong, but the plastic is sturdy so it's a free permanent bong) about half an hour ago. Took a typical number of hits for me (about 3-4 in a small bowl) to get me relatively stoned. I hadn't eaten much at the time, and I had just gotten home from a few hours going door-to-door marketing in the snow (It didn't feel too cold, though).
 
Anyway, I got back to my room from my washroom and was sort of half bent backwards trying to put some eyedrops in, and when after I finished one eye, my vision started to go and I felt a strange feeling in the back of my head. Then I told myself to breathe consciously, took a deep breath (and felt some clarity), then I straightened up, thinking that was that (THAT had happened before from that position with my head facing up). However, suddenly I sort of spaced out for about 20 seconds, unable to consciously do anything but sort of be aware of the fact I'm standing in my room. Then I snapped out of it, wondering wtf just happened.
 
I laid down on my bed for a while and I felt alright; not particularly high nor ill. I'm sitting upright typing this and I feel that feeling in my head a tiny bit, but that is sort of familiar from times being very high.
 
Any thoughts what that was? Thanks!!!!

 

Okay, but what can you tell me about the almost-fainting happening when I tilt my head back completely and why does being on weed bring these effects out?

I'm not really scared because obviously weed isn't fatal in any real circumstance but its unsettling undoubedtly