Sounds like your account was hacked and is being used to send spam. Check with other people you have listed in your email address book. Often times, it will be sent to everyone in your address book.
Notify everyone in your address book that those messages are not from you.
Contact your email service provider and let them know what is happening.
You can try changing your password and see if it stops. Otherwise, you may just need a new account.
Best of luck.