I've sent a payment, but it was sent not from my address, why?

This is because there is no "from" address. Cryptocurrency addresses are used to receive payments, but not to send them: there is no concept of a "from" address in cryptocurrency. You cannot send exactly your coins when making payment.

Think of incoming cryptocurrency payments as of banknotes which you deposit to your account in some bank. Say, you came to deposit $100 in ten banknotes of $10 each. Once you wish to withdraw your deposit, you won't get exactly that ten $10 banknotes back, but some other banknotes instead.

You can read more about it on Bitcoin Wiki