It is not always the apparent author of the spam whose account has been hacked. Your email address may well have been harvested from another computer's mailing list, and then used as a nominal source.
Your are right to change your password, 'in case', but if this happens again, then the probable cause is as sketched out above, not another hacking of your account.
Hence the desirability of blind carbon copying at all times and also of deleting ANYTHING that could possibly identify anyone other than oneself before forwarding an email.
Dave (a frustrated recipient of those John Smith; Brian Jones; old uncle Tom Cobley & all emails!)