Dying from Nothing!!

I was playing Nautilus with over 3000HP and 250 Armor/150ish Magic Resist against an almost full AD team; Jayce, LeeSin, Pantheon were all the enemy teams kills and dps. I was dying from under 20% total damage even at full health almost instantly even 1v1! Over and over they would kill me, but never ever total over 500 damage dealt, ever! Please explain Riot?