Amumu ult showing active when it's not

In my last game I played as Amumu. I was about level 9 in a team fight, used my ult, and was killed. When I came back, it showed that my ult was still up even though I knew and saw that I recently activated it. I tried using it again for the next team fight, still showing that it was active, but nothing happened. I only noticed this once in the game. The rest of the time when I checked I saw that it was showing the cool down timer.