If you haven't seen any unusual activity on your machine the chances are it's not you who is sending them but someone with your email address in their address book who has been infected by a virus/spyware and it is spoofing your address in the 'from' line.
As a precaution scan with some extra tools such as Stinger and Malwarebytes Free, both listed in the last link in my signature below. Suggest that to your friends also.
You're friends may have been fished. It is recomended to tell you're friend or family to perform a on-Access Scan. this will make sure they are not hacked.
Maybe, try malware bytes as ex-brit suggests..