Hmm, even though Outta Pocket's response still seems pretty solid, do you get headaches when you read/ spend too much time on the computer? Weird focusing things that seemed to be isolated events that I didn't take seriously ended up meaning that I needed reading glasses (with a plus prescription). If it happens in absolutely no other contexts then maybe it's the weed, but I would make sure of this...it took me a while to notice that my distance vision wasn't as sharp (I already needed reading glasses) and going to the optometrist meant I needed some minor vision correction, not that I thought it was in my head and would only happen if I was high/tired/etc.
Just something to think about...and, Outta Pocket's explanation also fits with the whole weed helping glaucoma thing, which is pretty well researched.