What the fuck is happening? My percepetion has decreased by 3, and some other stats have been adjusted by + or -1.

According to the manual

Your statistics will, generally, not improve directly through
experience or game play. You should plan on seeing the same statistics
throughout the course of your adventures, and plan accordingly.

I dropped all my gear in Junktown, and traveled to Shady Sands, and the stats readjusted themselves.
So what gives? I can understand stats go down if overencumbered or if wearing armour gear that block senses or restricts movement, but AFAIK there's no mention of any of this in the manual. Also the one stat that went up was Charisma (due to Garl's cool metal armour?).
I've not used any drugs, nor do I have the Night Person perk.

BTW, I'm using the Fallout Fixt mod.

Did you check if your character is injured (eye damage reduces perception) or radiated ?

Maybe you could go back to your home vault and visit the doctor there for healing, or use the doctor skill yourself or radaway.

I suspect the best solution is uninstalling the mod and playing vanilla (if the current GoG version is censored use a patch to restore the children).

I think I found out what was wrong. I carried an Irridated Fruit. After dropping it the stats slowly went up again.

This is strange, just carrying it shouldn't affect your stats.
That never happened with me in Fo1 or 2.
It seems to be a bug of Fixt, maybe send a message to Sduibek, to see what he has to say.
He has an acc here, but I don't know if he visits this site frequently, you'll probably have better luck in NMA.

Healing Powder?

It reduces perception.

Which comes back when you rest (travel = rest).

Edit:
Oops. No healing powder in FO1.

Fixt is too buggy, so I installed the TeamX patches instead.