It could also be something that was on the surface of the water and got caught in his bubbles. For example, if you haven't been changing it often enough, it could be build up in the water that's getting into his bubbles. As far as the tail biting, more frequent water changes can help that. Bettas tailbite for many different reasons, but some do it because of stress due to poor water conditions.
Here's a helpful link from this forum on water changes: http://www.bettafish.com/showthread.php?t=115758