Needing a new username for Minecraft Java, yet I already have one on my account

Hello!

I'm on Linux, after switching from Windows, wanted to try something new, just recently reinstalled the Minecraft Launcher, and whenever I try to play Minecraft, it asks for a new username. I tried to enter my old username, but "it's already taken". I tried entering one that wasn't taken, and it said "Failed to create profile", probably due to the fact I already have a username connected to my account. On minecraft.net, it has my old username I used prior to this. It hasn't changed, there's just an error going on, and I have no clue what it is. I installed the launcher last night, and just got onto it this morning.

Cheers!

:)