Change your password.
I'm no expert on security, but I seriously doubt malware is the cause of
this. People distributing malware generally target the much more vulnerable
Windows operating system, and I don't see how malware could help to take over
your E-mail account; either way, your password is needed. Change your
password, and possibly some other security details, and the spam ought to
stop.
Though while you're at it, you might want to change any passwords that were
stored in an E-mail on that account.
