You’ve probably done nothing, you just moved a bit more than usual in terms of range and your brain has gone ‘that doesn’t feel normal’ and protectively locked up your back as a protective mechanism. It does this by spasming all the muscles around the ‘suspect’ area which locks the whole area up solid to prevent any movement,
There are three ways to un freeze the spasming muscles.
1. Get a sports therapist / physio to use pressure point release to encourage the muscles to relax
2. Movement will loosen it up eg walking, gentle cycling etc
3. Just wait, the brain will switch off the muscles eventually.
NB You can do 1 yourself with a lacrosse ball, just slowly apply pressure to the most tense area and it will slowly relax. Leaning against a wall with the lacrosse ball over the most painful spot normally works. I have one in my desk draw and regularly loan it out to colleagues with seized backs…