Someone else, a spammer, is using your address as the "from" or "reply-to" address. There is not much you can do about it other than be patient and wait for them to move on. Inspection of the message source (ctrl+u in Thunderbird) will often reveal the true sender, or at least his general location. But the sender is usually not the spammer.

In the meantime, be wary of where you post your email address.