Are you playing in the same place you normally do? Same amp? Same room/part of the room? Same furniture? If not it’s possible you’re feeling a difference in resonance with nearby objects (i.e. if the floor used to sympathetically vibrate where you practice and now it doesn’t, your bass might feel “thinner” even though it’s not the bass).
You could play with pickup positioning too. I don’t know why this would have changed suddenly, but getting the magnets correctly aligned to the string can create a perceptible difference in volume. @Korrigan has a great post about that in Bass Setup - #46 by Korrigan
Just a couple weird ideas to explore! Other suggestions in the thread already are great too.
It can range from the 5th to 7th fret apparently. I’ve never actually noticed it on my Fenders, but maybe I just don’t mind it. Here’s an interesting article about that.