Doesn't sound right. If you were in management, I don't understand why you wouldn't have brought up your app at a meeting, and why you were unaware an app was already under development.
That aside, I'm surprised that you were fired, given your time with the company, and the supposed nature of your offense (creating a productivity tool).
I'm guessing there may be more to the story, for example, you accessed company files/code which you were not authorized to have access to, developing/promoting app on company time, or something of that sort.
Otherwise, you may have a case for wrongful termination.
I would want to know specifically what rules were broken and the exact reason(s) for the termination, and then I would probably contact a lawyer if I felt my rights were violated.
Whatever the case, good luck...
ADG