A trapped trapezius muscle

Given the data you provided (injured shoulder while doing martial arts, X-ray negative, no trapezius muscle can be felt), it is possible that your trapezius muscle was trapped when you injured your shoulder

Changing the location of the muscle might keep constant pain that does not want to leave. The muscle needs to be relocated in its normal position. Pain might be felt a few days afterwards due to inflammation of the muscle which might have occurred in the meantime. One fact is important: X-rays detect high density (bones) and low density substances (air in lungs), but is not the best imaging method for detecting smaller differences in density changes (muscles). An MRI might be a better idea in order to exactly locate where the muscle was trapped.

You might want to visit an orthopedist that would be able to examine the range of the joint movements, and when and in which position the pain starts and stops, as well as to examine the muscles of the shoulder.

Pain killers, muscle relaxants and physical therapy might be recommended in order to help the joint recover. Immobilization of the joint might be another treatment option if the doctor decides it is best for the muscle to rest in order to return to its normal position.