Dig in with your fingers around the shoulder and feel for any tight spots.
Likely, tweaked one ofyour rotator cuff mucles... possibly subscap or supraspinatus. Loosen everything up.. don't forget to check a bit into the arm, chest and around the scap as wll.
Note any tightness you have with a marked google image.
Any other shoulder articulations hurt?
http://www.exrx.net/Articulations/Shoulder.html