As you suggested, I contacted that email address and explained what's happened. I believe the same has happened to numerous other accounts as there are a multitude of similar reports the past couple days. This is the response I received, which did not address any of my concerns:
A support agent has suggested a solution for your Pokémon support ticket.
Thank you for contacting Pokémon Support!
If you have a Pokémon Trainer Club Account and received an alert that your email address or password were changed but you did not request a change, we strongly recommend that you reset your password immediately at the link below: https://club.pokemon.com/us/pokemon-trainer-club/forgot-password
*Note that when resetting your account password, you only have to enter your new password and username. Please leave the other fields for Player ID, Screen Name and Date of Birth blank.
Please do not give out your password or personal information on any game chat services, online forums, or message boards. Don’t let others test out your account by giving them your login information. Only log into Pokémon GO, The Pokémon Trading Card Game Online, or websites that are operated by The Pokémon Company International. Going to suspicious looking websites promising hacks or cheats for online games such as The Pokémon Trading Card Game Online or Pokémon GO is not supported and a violation of the game’s license agreement.
Please let us know if you have any further questions or concerns!
The Pokémon Company International Support Team
The following Customer(s) have been CC'ed:
Question #: 15182-956347
Date Created: 2/21/2018 12:08 PM PST
Is this really what can be expected? Not even an acknowledgement of my concern?