I changed my username and when I joined a server it treated me like a new account

Basically, I changed my username in Minecraft. I was bored with my old one so I wanted a new one. I had recently been playing on this survival server and when I hopped back onto the server after changing my name all my progress had been reset. I had no stuff, no experience points, no advancements, and I couldn't make my dogs stand up. It treated me like I had joined the server on a different account even though all I had done was change my name.