Ex post facto laws are forbidden by the US Constitution, both at federal and state levels. [1]

At least, at this time, with current interpretation- "the Supreme Court has explained that people must have notice of the possible criminal penalties for their actions at the time they act" [2] (See also Weaver v Graham[3])

This might be subject to change.

[1]https://constitution.congress.gov/browse/essay/artI-S9-C3-3-...

[2]https://constitution.congress.gov/browse/essay/artI-S9-C3-3-...

[3]http://cdn.loc.gov/service/ll/usrep/usrep450/usrep450024/usr...