Given that our appetite is controlled by our endocannabinoid systems, I suspect the nausea is a function of the wide swing and range of cannabinoid saturation that those receptors are seeing.
Studies have shown that when a neurotransmitter is in excess supply, that the body produces less receptors to receive them, as well as reducing the supply to conserve energy.
When we continually flood our system with phytocannabinoid, there is a good chance that causes the body to produce less endocannabinoid neurotransmitters, as well as less receptor sites.
If we flood our system before going to sleep, the lapsed time until we awake, most likely reduces the level of cannabinoids in our system below that which it would normally be if we hadn't been flooding our systems, because our body is no longer keeping up on its own.
I don't get morning sickness, but I do sometime experience short lived mild waves of nausea, that quickly go away on their own.