If outdoor mushrooms have dropped most of their spores and spores have a direct effect on body load then the result should be less problematic after they drop their spores. When consuming mushrooms from outdoors I would rather pin nausea on other factors like bacteria and other contaminants.
I was speculating about this for a bit, but the only thing I could come up with which might explain these effects only after spores start forming is that there might be a byproduct from meiosis/spore formation that has some influence. I am unsure if it is worth speculating about because I haven't noticed any effects like this. The worst body load I ever had was from store bought stones - I had an allergic reaction to them on my skin, and I think my belly/body reacted in a similar manner. No spores in there..
I'd guess that there are a lot of things that could cause nausea in the context of fungi, especially fungi that grew outside.
And when spores "drop," very large quantities end up stuck to the cap, stem, and gills so will still be there when we consume them if we don't do an extract.