Why did I pass out on weed?

This happened to me a long time ago and never was sure of to why this happened.

I grab a Vitamin Water then go to my friends place to smoke. We go into his garage and smoke 1 j. At this point, my tolerance is mediocre. I could smoke more, usually. But after this one, I noticed I had tunnel vision and couldn't keep standing. It felt so weird, being this the first time I felt anything like this. Next thing I know, I wake up from being on the floor. My friend said I passed out and whacked my head on the wall. I got a gash over my eye and had to get stitches.

Why did this happen? I was scared to smoke for weeks after. Of course that fear is long gone now. Was it low blood sugar? /shrug