Clare, it could be a number of things. Loading the spine with that much driving could be most of it. Even though I only had a one level ADR in 2004, it took me 12-18 months to recover to the point where I had painless driving. And it took almost 2 full years to feel "normal" again in my car seat!
Are you taking any NSAIDs? If so, what and how much per day?
