Not at all unusual. You will have been accidentally tricked into giving your password out on a site that possibly offered new emoticons or to show who blocked you. Something like that.
It's not unusual at all, you can change your password, but you may find it won't stop them, they could be sent from any address and still have your address set as the reply address to them.
If changing password doesn't stop them then they will probably stop on their own after a while, but by that time you may find you've been blocked from so many places with that address it's not worth keeping it.