I'm sure this is happening because of some stuff they spray on the plants.
I have a fairly nice tolerance and when i have days off i start smoking in the morning and stop late in the evening, i'd estimate i smoke around 3-5 grams a day of fairly potent stuff. 1-2 grams almost every evening
..and i have never had anything like this. I know heavy smokers and i have heard none of this from them.
I can get slight nausea/sugar crash if i haven't eaten well and i smoke something strong, but when i eat something it goes away. When i'm smoking on a full stomach, then there's no worries.
For me heavier using brings higher tolerance with it as well, meaning weed shouldn't mess up heavier users that easily. Strong weed can cause psychotic experiences on light smokers, but heavy users shouldn't be all that bothered, at least this is how it is for me.