According to TOSHI's blog, he is suffering from intercostal neuralgia. Intercostal neuralgia is a common condition that consists of abdominal pain caused by entrapped nerves in the abdominal muscles.
TOSHI states, that in his case, his condition was brought about by high amounts of stress and that he is resting now.

We wish him a fast recovery here at Xplosion. Get better soon!

Source: Toshi's blog, Wikipedia on intercostal neuralgia