Sorry, no. "Ex post facto" has nothing to do with increasing the punishment after conviction. It refers specifically to declaring something illegal, then prosecuting somebody under that law based on conduct that occurred before the law was passed. Regardless, punishment is increased after conviction and sentencing all the time. Violate the terms of your probation? Straight to the slammer. Start a fight in prison? You get sent to solitary. And so on.
Let's address some of the other things you may be misinformed about. This is not a bill of attainder, because it doesn't address specific people or groups of people. And no, the "group" of people who are in prison doesn't count, because potentially ANYBODY could be in prison for some other crime. If the bill stated that it only applied to black people, or people named Steve, then it would not be permissible. It's also (arguably) not a due process violation, because since it is a civil punishment and not a criminal one, the standard is lower than what is necessary to convict a person of a crime and send him to prison. You still need to convince a judge under this law, so there is still a due process issue. But the Supreme Court specifically did not address that in this case.
What's at issue here is ultimately whether the United States, given the authority to imprison somebody for a crime, can indefinitely extend that time under a civil statute based on a showing to a judge that it is necessary. This is not legally any different from the authority to put somebody in solitary confinement, or deny access to privileges based on behavior. I'm not comfortable with the ruling in this case, and I agree with Justices Thomas and Scalia that this is an inappropriate extension of government power. But please get your facts and legal terminology straight before going off half-cocked based on your limited legal understanding.