stress maybe?? when you are really stressed all of the muscles around the shoulders and neck can become overactive...try warm baths, muscle rubs, relaxation etc
alternatively perhaps its your pillow in bed at night... try investing in an orthopedic one...
also...watch your posture... avoid having a "poking chin" when you are out and about... that would put big pressure on your neck...
I beieve if one works from the top down, thru a chiropractor, one sees better and lasting results!!