Could be residual energy build up in the body. It hasn't let go from all the tension an anxiety yet. It has happened with my oldest too. Poor girl 
Makes sense. She was sick and tensed, so walking or biking tension away was no option. I used to massage her before bed, when she was tensed. But, she severely bruised her shoulder a while ago, all seven attached muscles afflicted big time. Massaging was not an option.
But, she is healing, maybe I can massage her soon again.