I know what you're going through; I still experience those moments every once in awhile. You getting blazed easier than usual is a reverse-tolerance, which also appeared in me a couple years ago. I used to be able to smoke an 1/8 like nothing, often ghosting my hits, then one day found myself blazed off a bowl. I think it was around this time when I started smoking by myself more often. When I'm around people, I don't think about the high very much, and I focus on the conversation and the experience, whereas when I'm alone, I'm forced to be alone with my thoughts and my concentration often strays to negativity. What if this, what if that. That's when the paranoia comes in. You said that in one particular instance of the anxiety, it occured right as your girlfriend left the room, so it has to do with where your concentration lies, no? Weed, at least for me and many others, is something that's best done with a head free of worries, responsbilities, etc, and once those enter your head, the high goes to hell.