Did you actually go into your sent mail box and see the sent mail there?
Just because an email shows your email address as the "From:" address doesn't necessarily mean that it was sent from your actual email account. It's possible for spammers to put
any From: address on an email.
As an example, I could write a script that would send emails to people with the From: address set as "
[email protected]" -- it doesn't mean that the president sent the spam ;) Smart email programs/servers are able to tell that the From: address is different from the server that it was sent from (this is all information that is in the email's header, which is generally hidden from you when viewing the email), and will then block the email as spam or put it in your junk box.
One of the problems is that if someone uses your email as the From: address in a spamming script, if they send the emails to non-existing email addresses, then you'll get the bouncebacks (i.e. emails saying "This email address doesn't exist.") If you get those, it's nothing to worry about -- no one has hacked your email; it's just a spammer using your email address in the From: address to try to trick people into opening the emails.