Apple ID loged in via iMessage NOT ME

Hello,

I keep getting email alerts that my account has been accessed via iMessage from another device, that isn't mine. I have changed my password numerous times and aIso enabled two-factor authentication. I do not get an alert for the two factor authentication whenever I receive the e-mail about someone logging into my iMessage.

I have also checked to see what devices are logged into my AppledID - and it only shows MY phone.

Here is ONE of the messages:

Dear ,

Your Apple ID (my email) was used to sign in to iMessage on an iMac 12.5" names "macen (weird symbols) iMac"

Date and Time: September 19, 2019, 4:09PM PDT

If the information above looks familiar, you can ignore this message.

Then the rest of the e-mail...

So HOW can someone log into my iMessage... without me approved it with two factor authentication... AND how do they keep doing this after I change my password... etc.

HELPPPP!