Well, to be honest, a hernia does not just go away like that. That does not mean you should worry. What probably happened is, like you said, there was gas build up and it caused pressure and bloat. But, that would cause more generalized distention if there was no hernia. Yours was very localized and with it being at the stoma site, it was acting exactly like an incisional hernia, and the stoma site is a common location. It is typical for hernias to slip out and in. That is a good thing as it prevents bowel strangulation.
Yours is quite small and can become less noticeable over time as your abdominal muscles get stronger. I would not recommend any heavy weight lifting though. The defect will always be there without a repair. I had an incisional hernia the size of a grapefruit and repair was optional for me as long as I was not having pain associated with it.
Jan 