Hi,
At a time when I was smoking regularly, it came to a point where, come bedtime, I'd see things on the back of my phosphenes, images within the clouds so to speak. Sometimes I could make out places, such as the living room etc, othertimes it would look like silhoutes of people, it was quite distrubing actually. Sometimes I would have to open my eyes to check that they were closed because, laying in the dark, it was sometimes hard to distinguish.
I stopped smoking because I figured out that it was the weed that was causing sleep paralysis for me. I don;t know how much anyone knows about sleep paralysis, but the first time it happened I was laying in bed just drifting off, next thing I know my mom comes into my room and sits on my bed and starts talking to me, it was then that I sensed that it wasn't my mom, but an imposter! At that moment I turned to look at my mom and instead saw a 7ft cloaked figure (think sterotypical 'death') and it throw my blanket over my head. At that moment I felt a number of hands start pushing me through the bed and down...down....down, I can openly describe it as though I was being pushed down to hell, thats what it felt like.
Next thing I know I shoot up out of bed - I was in the exact same position as before the weird stuff starting happening, albeit I was covered in sweat. I was just like WTF was that?? The shocking thing about it was how seamless the transition was. Anyways, I had other experiences after that which involved things sitting on my back, voices whispering as if they were right by my ear etc etc. The point I'm trying to make is that I certainly think some people can be receptive to the hallucinogenic effects of cannabis and things like the OP described can occur.
It virtually stopped once I stopped smoking, the only other time it happens is when I am severely sleep deprived.
Methtical