Pain on the ball of the foot is common and can be caused by a number of different conditions. There are many different structures in the ball of the foot which any one of them can be affected leading to so many different potential problems.
http://www.foothealthfriday.com/pain-on-the-ball-of-the-foot/ (Foot Health Friday)
http://www.foot-health-forum.com/forum/showthread.php?t=118 (Foot health Forum)
http://www.foot-health-forum.com/forum/showthread.php?t=79 (Foot health Forum)
http://www.foot-info.com/ball_of_foot_pain.php (Foot Info)