It could be anything - from soft tissue to bone or joints. Soft tissue injuries (tendons, muscle, ligaments) normally do not show up on x-ray, but doing an x-ray can rule out if there is a skeletal injury. If nothing pops out as abnormal to your vet, they will probably recommend a period of rest with anti-inflammatories. If no improvement, then an appt with an ortho vet will be necessary.
Our girl had a torn ligament last year and had surgery to correct the problem. The thing that concerns me about what you wrote is your dog sleeping a lot. Ours had normal energy even when injured. Hope he is not in a lot of pain, is his appetite normal? At least tomorrow you know he'll be in good hands and you won't need to worry.