People receiving Spam from me, but nothing shows in my “Sent” folder

A couple of weeks ago people received spam supposedly from my account. I checked my "Sent" folder but nothing was sent from my account. I went ahead and changed my password anyway. I also created a fake contact at the beginning of my list so that if spam were being sent to my contacts from my account, that one would bounce and be an immediate alert to me. This week the same exact thing happened. People are receiving spam showing it to come from my account, my fake contact did not produce a bounce, and my "Sent" folder does not indicate that the spam was sent from my account. How is it that people are getting spam emails purporting to be from my account when I see no evidence that anything was sent from my account.