It wasn't actually sent from you. It's called spoofing and it tricks a lot of people. Look at the extended header of the email and you will usually see your real sender. To be honest, any email service out there has the power to change the FROM address with a lil knowledge.
There are some great tips and advice located here:
*www.cert.org/tech_tips/email_spoofing.html
on how to deal with spoofed email.