I paid ACTUAL MONEY for my username Cinnamon. I just logged in after a hiatus and it’s been changed??? Excuse me? Why.

If you were gone over a year it becomes open for people to use. Paying actual money doesn't stop this.

You ARE however able to do a rename for free due to this. Same with pets if any were taken.

If you left for over a period of one year, your UN and your pets' names are returned back to the user base. Prevents hoarding on unused accounts. You can change your name for free and your pets' names for free as well.

You were also probably given plenty of warning on the email that you used to register your account about this coming.