For whatever reason, 4 years ago, I became super sensitive to aspartame. It would make me hyperalert like a more subtle version of caffeine. I had insmonia for random days. It took me forever to isolate it to aspartame, since I've never had a problem, nor suspected there could even be a problem. I don't have pku but it is what it is. Sucralose, stevia, regular sugar still fine. Bodies are weird. I still believe in science but the older I get, the more I give credence that in some people things just work differently. I don't just jump to the conclusion that they're making something up just because official scienctific papers say it's 99.999% safe.
I'd bet there are digestion processes that "start up" in response to taste. Maybe your body detected that stevia was regularly "starting up" one of these processes and then withholding the expected glucose spike, and got upset about the pattern
That's phenylalanine. Its metabolites are interesting.
One of them is tyrosine, which itself is the precursor to dopamine and adrenaline.
Another is phenethylamine. That's a fun class of drugs.
https://en.wikipedia.org/wiki/Substituted_phenethylamine
Phenethylamine itself becomes N-Methylphenethylamine, not at all dissimilar to its structural isomer, A-Methylphenethylamine, better known as amphetamine.
How much phenylalanine is metabolized into each of these is complicated and depends on a lot of things, but sounds like you did something to cause it to be more.
I mostly stick to soda water (only carbon dioxide, no flavorings) these days.
and there are plenty of instancess where gut microbiome changes are known to be impactful. my partner got SIBO which caused lots of problems for a few months and she was basically unable to eat anything.