If it makes you feel any better, if it was a neuro-degenerative condition you'd notice other symptoms than just "my English isn't as flawless".
There are a lot of potential factors, my best guess: you're tired, stressed out, and the otherwise small fail we all do once in a while spiralled out of control as it happened in an area near and dear to you: language.
In other words, you had a perfectly human error due to stressing out, which scared you, and made you go into hypochondriac mode. It's a lot more common than it sounds.
Another possibility involves the fact that you're, at least, bilingual from what I can tell (profile says you're from Finland). As someone who is also multi-lingual, I've had something like this happen before. If you've been using one language considerably more than the other, it's perfectly normal for the lesser used language to get a bit "rusty". Not "I can't understand it anymore" rusty, but "I no longer have half the dictionary in my head" rusty. It happened with both my native language (Portuguese), since I started using English predominantly, and happened with English for a while when I focused more on doing college essays and works (which, as you can probably guess, are in my native language), and used English less.
Either ways, don't stress too much. Great thing about the human brain is that we "forget" a lot less information than we think we do. We mostly just store it outside of our "work memory" for convenience. A little brushing up and you're back on track. Browse some literature if you feel the need to dust the old lexicon a bit.
