I'm surprised nobody knows about this. Some of the cannabinoids cause a temporary drop in blood pressure, particularly if you get really stoned or have low blood sugar from not eating recently. If you are standing up it will be worse, the only way to make it better is to lay down as soon as you feel yourself getting lightheaded. You can even elevate your feet after you are laying down. Sip some water or some juice if you haven't eaten lately. This is called postural hypotension, many medications can cause it.