There aren't any Bullocks in the 14th Mississippi Regiment. Perhaps you meant the 12th Mississippi. These are probably different men since at least three Thomas J. Bullocks lived in Mississippi -- one each in Pike, Calhoun and Marion Counties.
Sometimes soldiers were temporarily assigned to a different regiment if unable to report to their regular command because of capture or interposition of Federal forces. For instance two companies composed of survivors of Alabama regiments captured at Fort Donelson served with the 33rd Mississippi during the Kentucky Campaign.
Other than these kind of circumstances it would be strange indeed to have a soldier detached to a command other than his own.