Dogs that have elongated backs often have back and disc troubles and often suffer from paralysis.
Your vet can tell with an x-ray if there are problems. I suggest that you make an appointment to rule out a bigger problem.
Until you can do so, you should keep him from jumping as much as you can. I know that these dogs are high energy but you can still place stools and boxes so that he can climb up and down instead of jumping.
You would not see this happen with chocolate consumption, particularly if it was a week ago.
I hope that this helps!
Let me know if you still have questions.