Same copy of my website with a different domain name

Hello,

I found out that there is a website same as me, same content , same users jsu a different domain name (I even logged in successfully with my admin login to the fake one but i couldnt make any changes to my website from the fake domain.) .. and any changes I made to my website, it takes place instantly in the other website,

Im wondering what could be happening ? is my website is hacked ? and how can I solve the issue ?