I've had on-and-off back issues too.
About 15 years ago I was riding bareback and fell off, flat on my tail bone! I never went to a doctor and it took weeks to heal. I was in high school at the time and would get in and out of those hard desk chairs like a little old lady. I know I must have cracked it if not flat-out broke my tail bone.
Fast forward about 15 years, and I bought a new saddle. My tail bone starts giving me terrible pain when I ride. Eventually I learn how to change my riding posture as to not put as much pressure on the tail bone. Also, longer stirrups seemed to help. I kept the saddle because it fit the horse. :roll:
Then, last year I just had to try a different saddle. Same horse with saddle fitting issues. I buy a Reinsman Flextree. Fit the horse fine, but it killed my back! I got the saddle in January and didn't realize it was the source of the back pain right away because the weather was crummy so my rides were short. Well, several months later my back was so bad the doctor gave me a shot in my lower back. What a relief! I sold the saddle. Then 6 months later, I had to get another back shot because the pain returned. I think that saddle did some lasting damage to my spine.
I was good for another year. Now, I have a new horse, a Foxtrotter, and who would have thought, but my back is acting up again! And they say gaited horses are good for your back.
The only thing I can think of is because she is narrower than my Mustang, my saddle sits on her lower and changes my posture. I find that longer stirrups help my back, but that might be an individual thing, so I don't know if it will help you. And my back hurts when I have to lean back. I do better with a saddle that makes me lean forward slightly. If it's level or downhill, my back hurts.
So I don't have much advice, but you are not alone!
Well, one word of advice, if you do start riding again, try a different saddle to see if that makes a difference.
Back pain is horrible, because it can start out minor one day and then get bad fast. There were times when I couldn't sit normally or sleep comfortably.
The shots (I think they are depo-medrol?) really helped, but because they are a steroid, you can't take them very often. I find the main thing that brings on my back pain is a saddle that affects my riding posture negatively. Ugh!