I was FLAWLESSLY hacked on RuneScape? How did this happen?

Today I signed on RuneScape, went to my back, and realized that my full rune armour, rune pickaxe, rune hatchet, all my runes, and all but 70gold was missing. My bankpin wasn't even broken into, my password wasn't changed, and my recovery questions were in tact. Litterally NO ONE knows my password or pincode and isn't there a trade limit? My password is deffinitly not easy to guess. I forgot to check the last time I signed on date before I signed out so I can't figure out when this happened.. But how exactly could this have happened? :S