My question involves labor and employment law for the state of: Carolina

I'm a city employee (parks dept) for a large city in South Carolina. Recently, another department's (water) manager gave several statements on a news interview. I disagreed with what she said and sent a private message, while not at work, to her Face book account. The water department's manager has now contacted the park's dept's manager and notified my manager of the situation and now a disciplinary investigation is on going.

I wanted some people's thoughts on this situation. Can the off duty use of private messaging be subject to discipline? This was never posted to a story/public view and was just an instant message. I know it may be a long stretch but, does the fact the water dept's manager has contacted my manager to initiate a disciplinary investigation constitute "retaliation"? Does our 1st amendment give me any protection in this case? The message was not threatening, vulgar, etc. It was just a message saying that I disagree.

Thanks guys! Please let me know if I may clarify anything.