Hey there: I think you already have your diagnosis here: OCD. I have just figured out today, because of another posters' suggestion, that I most probably have OCD "Pure O" type. Which means I have obsessive scary and bizarre thoughts that I can't seem to control. So, my take is this:
I've smoked pot on and off for years--in fact I stopped for a long time, because I felt really disassociated from reality--not in a delusional way, but that feeling you describe where you feel like your walking around in a movie and it isn't real--perfect. Also, I found that pot really exacerbated my obsessive thinking which could sometimes bring on a sever panic attack.
I started smoking pot again a while back, and I remember going through some mental mojo to get past some of that. However, when I smoke pot, I always get that sense of unreality--I wonder who the hell my husband really is and how I got here ( in this house with this person, in this life..) none of it seems real. Personally, I think that's your OCD talking. Your basically tripping your self out. Everyone has that feeling, but it takes a person with obsessional thinking to really turn the experience into a bad trip.
In fact, I've always suspected that that is what is really behind the whole "bad trip" scenario in regard to weed and or hallucinogens. You start obsessing on a thought and it becomes a nightmare.
I have experience in this dept...trust me.
That's my two cents!