The website you used has a brainwallet section. Did you by any chance use that with a "password" to create your private key? Because considering how it is producing your key using a single SHA256, the result is going to be the least secure key and the chances of the key already existing in some database and the funds being stolen is pretty high.
https://github.com/coinables/segwitaddress/blob/master/index.html#L426