no melee damage??

Hi, i need help with something, in singleplayer, if i want a zombie survival quest or something the attacking zombies/antlions/metrocops+stunstick dont do any damage to me. all i hear is the miss sound, same with antlion guard, it runs through me as if i didnt have a hitbox. and metrocops are the wierdest really cuz they run towards me, hit me once (miss) and then they run the last bit and stay INSIDE me so i cant move. i can only be damaged by bullets or explosives. ive tested npcs, they take normal damage from melee attacks. very strange, help please :S

reinstall gmod. Your game is screwed up.