Somebody Hacked my AOL Instant Messenger

I received a letter of terms of service from AOL Aim regarding a message I supposedly sent to a contact of mine. I never sent the message and would not ever send a message like that. Someone- somehow has used my email address WEW17@aol.com to send a message. As a coincidence this particular address has been sending out messages to my contacts that I am not receiving any mail until after 12/10/2010. I have had contacts from many friends regarding this, and I would like both of these matters investigated at once. How does somebody use my email address to send these fake messages through AOL? It seems that they have hacked my instant messenger account.