Your best and safest bet would be to go to the vet and find out what the problem is instead of worrying about it. Depending on the breed, there are a lot of problems that can arise concerning backs, legs and hips. This could possibly be the beginning of a problem in the future and if that's the case you need to be heads up about it.
My friend had a Pekinese whose back would go out of place and she knew exactly what to do for him.
Good luck to you and yours.