BUG during GAMEPLAY - Swain R Grey Out (can't use it)

I was playing Swain during ARAM match. By the end of the match, I noticed swain R was grey out but I still continue to play. Then during the next group fight, I was in the middle of all opponents and couldn't use Swain ultimate. I kept pressing it but it's not activating the ult then I got killed. I'm not talking about the second activation where you have to accumulate a certain amount of damage. I am talking about the first activation. After I respawned the R was still grey out until like 15-20sec later then everything when back to normal.