Assuming you meant 'Level' and you entered a Hacked Lobby e.g. 60K XP per kill then in one game you could have fully Prestiged a Soldier. It happens. Now as long as you weren't the (Host Hacker) you have nothing to worry about, it cannot be undone, so move on to the next Soldier. If you play long enough you'll eventually run into one of those lobbies, it's not a 'bug' it's a 'hack' that causes this problem.
If you know the host then Report them and leave a lobby if you're receiving unusually high XP.