The body is a funny thing. Pain doesn’t always manifest where there’s damage. Years ago, when I had ulcers, it manifested as back pain. Might not be what’s happening here, but it’s something to consider.
See a doctor to be certain. That being said, first few extended fasts I did. I got bad lower back and ham string aches. Like really deep bad muscle soreness. They eventually stopped happening. There are tons of videos online that explain the multiple different reasons this can happen. Based on my personal experiences I think it was tissue repair and cleaning in overload etc, based on how badly I had treated my body prior to those first big fasts.
I’ve started doing longer fasts recently (going from 18 hour fasts to 24) and during the first week of this transition. I got this back pain so intense, I wished I was dead. I’ve also had a similar back pain experience once or twice before (weirdly, those were also during a time when i had just taken up fasting). I never really had any back injury and these are the only instances of my back pain.
I think that my back pain was related to bloating and gas. I remember I was really bloated the time I got the back pain recently. I had done some squats (they usually help me with the bloating) and in a few hours the bloating was gone and so was the back pain.
I hope you don’t go through the back pain again, but if you do, notice if you’re bloated and you might just have your answer.
Good luck!