My email address keeps sending out spam to my address book

I don't know if this is malware or not, but my friends in my Yahoo address book keep getting emails from my email address about free pharmacy products, etc, that I never sent. It happens about two or three times a week. I've scanned my computer for spyware/malware/viruses, but I haven't found anything. Can someone suggest something? Thanks.

Only suggestion I have is to change your Yahoo password.

That's what I thought, too. I just hope it's not some automated system that's captured our computer passwords, too.