You are likely consuming a trigger food that causes systemic inflammation, which is why you notice a reduction in back pain during your periods of fasting. Discuss an elimination diet plan with your doctor, do some research on it yourself as well (I managed to do it without much guidance). It is VERY depressing, a lot of people struggle with it, but if youre already managing some 24hr fasts, you will find it is well worth it to eliminate that back pain.
There was a point where I couldnt even bend over, it was too painful. Figuring out my trigger foods along with exercise, it is now a distant memory. Nothing helped till I discovered what foods caused inflammation FOR ME.
You could message me for any sort of advice on this, I have been through countless doctors before along with pages of medicine—it doesnt hurt to have a helpful ear along the way.