I have had several transactions on it, and I currently have 0.64 BTC in it O_O
Are you sure you're not mistaking? Maybe you have two blockchain.info accounts (i presume this is your wallet, since you're talking about the blockchain, and about logging in). I cannot see a single input...
This means it's not the work of a hacker, virus, keylogger,... The address just never had any inputs, hence it is empty.