My guess is that it was the pistol combat medal. I am not sure if they show up instantly. So maybe you killed an enemy 10-30 seconds before and the medal just popped up when you teamkilled a guy. That would be my explanation. I'm not too sure if the medals need some time to show up on the screen. I just think (but I'm totally not sure) that the last time I got one, it took little bit of time.