Q: "Your Apple ID was used to sign in to iMessage on a MacBook Pro." email...

Today I received this email:

It was sent from "noreply@email.apple.com" and everything seems nice and official, but I'm scared for what this means? I don't own a MacBook nor does anyone around me so it wasn't me. When I picked up my phone I also had the same popup message there.
I went in and changed the password and the only devices it shows as being logged in are my own phones, but now I'm very paranoid. Was changing the password the only thing I can do to make sure my account is secure and no one is staying logged in? I can see no signs of anything being done other than from that email but I was just paranoid and not sure if there was anything else to do.

Thank you!