Activation lock when I'm the only owner that my iPhone ever had, asking for my AppleID
Activation lock being on is a good thing; it means that your device cannot be activated on a different account or carrier until it's disabled, usually at the time you sell or otherwise dispose of your device.

I'm not sure what would cause it to randomly pop up like it did - the first (and really only) thing that comes to mind is your carrier making some adjustments on their end that caused your phone to need to be reactivated, but as you saw, no data is lost during this type of activation. It's also possible that something was corrupted on the device that required reactivation with the carrier to resolve. You would also see this screen if you put in a new SIM card.

You should change your password for your Apple ID to be on the safe side, but it's unlikely that any third-party, aside from your carrier, was involved in this incident.