I have back pain problems right now that go away only when I stop exercising and stretching. I found this out when I broke my ankle and couldn't do anything. Back pain gone!
I love how everyone immediately goes to "you were doing it wrong". Gee thanks. It could also be you perform at a very high level and your body is aging, you have an underlying condition that had never had a chance to fully heal, or many other nuanced reasons.
So, in other words... he's doing it wrong. Perhaps "wrong" is "pushing too hard given age or some underlying condition", but the fact is that exercise and stretching should never make you worse off... assuming you're doing it correctly.
Sounds like you need to stretch correctly after exercising. I had the exact same experience, and it wasn’t until I began focusing my stretching on my hamstrings that I saw immediate and significant relief. It’s the holy grail to a back pain free life for me now.