Could be just all tensed up. That is how we protect ourselves when we have anxiety. Ever see a cat that sees a dog? The way it goes up on 4 paws and the hairs stand up. That can be us at times. So once we relax the muscles can still be sore. As you say, it comes and it goes. But as you are aware of it you focus on it. It can become something bigger. So just try and take your mind off of it. Then you will be less aware of it.