Are you by any chance taking any medications of any kind? Besides the weed, that is? What you're having sounds a lot like "orthostatic hypotension," which means you have a rather dramatic spell of low blood pressure when you stand up quickly. Because enough blood isn't getting to your brain when you stand, you experience the altered consciousness, strange hallucinations, visual changes, and even falling/fainting for a period.

Standing low blood pressure is fairly common, and it's a common side effect of lots of medicines. And since weed can often relax blood vessels and lower people's blood pressure, it even makes sense that that might cause it. Temporary low blood pressure's not dangerous in itself, except that most people do want to make sure they're getting enough blood, and consequently oxygen, to their brains when they stand up because there's a risk that you could faint or fall against something like a coffee table and injure your head in the process.

Might be worth cutting back slightly on the chronic to see if that makes a difference. Then if the symptom continues, it might be worth asking a doctor about it and/or looking at any other meds you're taking.