Tulip mentions a weak core. I tend to agree with her: strengthening your core is a good idea. Also, when is the last time an instructor checked our set-up (not just the handlebar height, but the fore/aft and the height). Might be worth getting set up all over again. Settings for people can change over the years. I'd recommend that too.

Good luck. I had back surgery some years ago (L4/L5 discectomy) and it worked out well for me. I hope you don't have to go to those measures (it doesn't sound like it based on the kind of discomfort you are having).