Why did I get a notification on my phone that says "Your Apple ID is now being used for iMessage on a new iPod Touch"? I was very confused when I saw this notification. I've been asking my mom to get me an Apple Watch and I'm not sure if she is trying to surprise me.
If your mom got an iPod and knows your Apple ID, that might explain it. Does she know your Apple ID?

This can also happen with existing devices already on your account if it needs to re-sign-in to iMessage.

Go to Settings > iCloud > tap your account name/photo on top and sign in > Devices. That will show you all of your devices. Tap the new iPod and answer your account security questions to see details about it and locate it on a map ("Show in Find My iPhone...") provided that it is powered on and set up for Find My iPhone.

Or you can open the Find iPhone app on your phone if you don't remember your security questions and want to locate it. If you see it's on and in your house, you can make it ring to locate it, but that will get rid of the surprise I guess. Or if it looks like someone else somewhere else is using the device with your Apple ID, you can wipe the device and remove it from your account.