I can't log into minecraft because I don't know my email?

I bought minecraft years ago, and I ALWAYS logged in using my username instead of my email and since it's been so long I have no idea what i used for an email…
A month or two ago I tried logging into minecraft the way I always do, and I got a message saying:
"(UserMigratedException: Invalid credentials. Account migrated, use e-mail as username)"

I do not remember ever migrating the account and I tried using every email I have ever remembered having.
How/Why did this happen? What do I do?

Minecraft accounts have been changed to mojang accounts for a while, go on the mojang website to find out more