German Shepherd Back injury Question

My dog always runs,jumps, and chases things. The past two days I noticed he was laying around a little more than usual, and he did not want to jog when I took him outside.

When I got back from a walk with him, I patted him on the back and he yelped with pain. He is very sensitive to his lower back area when you touch it.

He still runs outside, chases things, and jumps off the deck when he goes outside, but just looks to be a little slower with the activities. Could this just be a sprain, a pulled muscle, or something more serious? Thanks.