Email changed on account

I have preivously been hacked twice. Both times were when I got complacent with not having my authenticator on my account and I shared a password with other sites. That is not the case for this instance at all. I was using a password that is used nowhere else and an authenticator. Recently I switched to the dial-in authenticator. In the last few months I only log in from a single location.

Well I wake up this morning to my email on my account has been changed to a hotmail address. I've just completed a full scan of my system with both Malwarebytes' Anti-malware scanner and Symantec Anti-virus with no result of anything unusual.

My main email address provider, Gmail, keeps track of IP's that log into my mail and no unusual IP's have accessed my email account in the last 24 hours. Also the only email I got indicated that the email address had been successfully changed.

#1 I guess the mobile authenticator failed to prevent this hack attempt. - There is absolutely 0 chance this is an inside job unless I have someone living in my attic I don't know about.

#2 I'm absolutely flabbergasted how this is happening considering I'm constantly checking for keyloggers, I was using the actual authenticator before and switched to the dial-in about a week ago. I was using a different password than anywhere else.

I'work in IT so I do understand how these things can happen, but I think a lot of people underestimate the ingenuity these hackers have at getting access to your account.