New Server hacks? UUID Changer? Help!

Hey guys today my server got griefed by... Myself... and i nothing know about this.

So i have an bungeecord netwerk with many servers.

A new player joint the game and comes online With my UUID and the Name "smillingDeath"

But the Real name from the UUID that the player has is "SimbaZockt"

So he tooked my UUID for his name. And gave herself Op + Permissions ex "*"

How does that work? and how can i fix this?

http://prntscr.com/6l98xe

....

Sorry for my bad English.. Im German.