Retaliation and or more

A policy was created a few weeks ago forbidding a specific action. That action I stated I might have committed in the past which initiated the birth of this policy. Is it legal for this company to reprimand me for breaking this company policy when it was not in place at the time. I refuse to sign acknowledgement of this action resulting in their claim I was not following company policy when the action was not against any laws or company policies at the time, resulting in my termination.

A comment was also made during this acknowledgement meeting regarding my investigation for another place of employment. "At least we were willing to meet with your investigators for other employment" was said to me by a person in Human Resources. The fact that this is irrelevant to the meeting leads me to believe writing me up is in retaliation for me looking for alternative employment and termination is retaliation for not acknowledging I broke company policy that again was not in place at the time.

Are there any laws protecting me from these circumstances namely retaliation and new policies allowing reprimand for past actions?