My young lady friend - don't tell the wife! - gets this. She needs a smaller guitar. She plays a dreadnought, but is quite petite. The awkward poisition she has to play in makes her tense up and this makes her ache.

There must be loads on the net about musician's aches and pains - it's a long recognised problem. But try to find a comfortable position and relax... Works when playing an instrument, too!