i have a 6 year old dachshund myself. their backs are notoriously bad because of their long bodies. they don't have any extra vertebrae to compensate for that long body. over the course of their lives, they jump, roughhouse, etc., and it degrades their back. it happens sometime.
that's why making sure they don't get overweight is important for them. good luck with the pup. here's mine...he thinks he's people