someone using my apple id & phone number for iMessage and FaceTime on a new iPad

Why would I get this popup message on my iMac? I have an iPad but not using it, I am currently using my iMac desktop right now. The message also says = If you recently signed into "Patricia's iPad" you can ignore this notification. I wasn't signed in on my iPad when I got this popup message on my iMac. That popup message only allows you to click on a button that says OK but it doesn't tell you what to do to keep someone off your iPad...please advise? I took a picture of the popup message =