Having just read this, from what you've said, I think you were within the rules, you just ended up with a very odd situation due to tile placement. The only tiles in your current plague zone were the outbreak, one flea and a previous outbreak, which is immune. As a result, there were no legal options for flea placement, and you had an outbreak which could not expand.
This would have been resolved when you either placed more tiles around the outbreak or drew another outbreak tile.