Email spam issue

I am not sure where to turn for help on this.  I have been constantly getting email from my screenname that I know I haven’t been sending.  I have changed my emial several times, but to no avail.  I have done several scans, but found nothing, so it couldn’t be a virus or trojan.  Is there another way this could happen?  I have checked with the site of my email provider, but to no avail.  How can I stop this?