My ult did and did not go off cooldown as Cho'gath. [GAMEPLAY]

I was playing a game as Cho'gath midlane and I was doing extremely well, however, at one point I went to ult the enemy Annie and my lux's auto landed at the exact same time. This resulted in my ult going on cooldown, but I got 0 stacks and when I held ALT and clicked on my ult to ping it, the game said it was ready to be used. I have a video of the recording. If you would like to check it is my most recent game as Cho'gath. https://youtu.be/bvGaf5hqGeE