It was in Madness, the only 2 opponents were at the bottom of the map capturing the flag. So it couldn't have been an opponent that killed me. And the text in the top right corner of the screen said that my team mate killed me. Normally it shows 2 names: one in red and one in blue, and now both names were in blue. So it really was someone from my team.

Another thing i have noticed is when i'm playing in a team (deathmatch or capture the flag), someone from my own team can kill me, while i can not hit him. How is this even possible? It this also a cheat or something?