I have a theory. At least it works to explain how I feel, and I've noticed this a lot.

What weed does for me (besides getting me high) is increasing the sensitivity of at least some senses. I can hear and track sounds better while high, for instance.

My theory is that this increased awareness is the cause for the paranoia when for instance I'm walking back home while high -- I can hear the wind clearly, my own steps, the water tripping on the plants etc, and maybe that's where it comes from -- you're dealing with more (and unusual) information which is not "registered" as a pattern.

Dunno if that's the kind of paranoia you're talking about, but that's how it goes for me.