Take the union rep in asap to find out what's up. Nursing politics are at work here...or at least that's what it sounds like.
It could have been a patient complaint or a staff complaint. I'll wager to say it's something that they have blown way out of proportion.
In my state you can record a conversation without the other party knowing. I would take my smart phone and record the whole thing as well. I just recorded a very important meeting with something I'm involved in. I just went in and sat down and fiddled with my phone and when the meeting started I put my phone down by my writing pad on the conference table. It was an app that can record while the phone is turned off (I love technology). It's not that important but you would be surprised what you hear the second time around...and it's not legal in all states; only a few.
Oh how I wish all bosses and management had our backs. All this bickering and policing over details for what. You must be good or they wouldn't have advanced you to days. Something smells afoul; keep us posted. If you have any buddies on the night shift, I would give a call to see if they heard anything.
Remember; you have rights, so don't make any rash decision in there. If they demand something of you tell them you'll need a few days to think it over; if they put you on leave or suspend you get all the info in writing and don't sign anything. In these types of meetings it's best to listen and not say too much until you are better prepared. Let the union rep talk for you. You don't need a lawyer when you have your union rep.
Go get em!