Is it legal to have job threaten after I had called Corporate and complained.

I called corporate, about insurance not going into effect as told it was, they took care of that. Also complained that I was not told that I would be working mandatory overtime every weekend when offered job. Was offered the hours of Mon-Fri 4:30 pm to 1:30 am. I accepted the job. Now after I had talked to Corporate, was called in on 11-11-09 to Plant Managers office, said that no one liked me there, that I had forged reciepts,(temperatures)didnt help others, and had left early on Sunday overtime without telling anyone. Also was mentioned that I should have talked to them and not corporate. All the statements that they asked about were not true, this was a threat. Was told that a wise man had told him (plant manager) that dont say anything that you wouldnt want your mother to hear, and also a wise man knows when to keep mouth shut. The meeting was held with the lady in charge of HR. It seemed that the basis of the meeting was to shut me up and not for me to call Corporate or their clients again.