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.
