Sesamoiditis is a common cause of pain under the medial side of the forefoot. It probably is not really a true sesamoiditis, but an inflammation of the peri-sesamoid tissues. The most common cause is an overload of the sesamoid bones.

Resources:

http://www.foot-health-forum.com/forum/tags.php?tag=sesamoiditis

http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=2504

http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=34317