Can my manager fire me for a Facebook post?

[Blaise]

Recently my manager came into work and was having severe pain, so she left for the doctor. I posted on Facebook, without using her name or the name of my employer, and just stated that she wasn't feel well and I was concerned. She just called me and freaked out, threatening to fire me if I posted anything else about her again. I work for a union employer and I've been there six months. Can she actually fire me for posting a status about it on social media?