Receiving spam email from my own iCloud account and my own IP address
I believe I've figured this out. This is the well known Gcal spam, but in my case, at least, and I assume @derpymcderpson's case, the Google calendar is linked to Calendar.app (formerly iCal). That means that an email alarm from Calendar when the Gcal spam is added sends an email from your primary email account using Apple Mail (or, more specifically, MailServiceAgent). No hacking. Just unfortunate behavior. It's also, fortunately, why it only goes to your own Gmail account.