Multiple Discord Servers hacked by same person

Recently I have been working on a Discord BOT, I am not a very skilled programmer and the BOT can only respond to messages. recently 3 Servers I am either in or own were hacked through the BOT. The user who hacked the Servers goes by the name "MutanPlex#0815". I know this Information as luckily On one of the servers I'm admin in had Mee6 who added this unknown individual and I saw this person only for a second on one of the servers I own + In one that wasn't hacked the BOT had created an Invitation. Luckily the user in question only hacked 3 servers and Backups for most of them existed but I am just incredibly mad and confused on how this has occurred,