Sounds to me like someone was trying to get rid of you. I would look at the day, the time, the charge nurse, staff nurse then go to the DON for the facility. Only you know who you told you where going to lunch and if they ok'd it. When in doubt always always ask the head nurse before..anything..if you so much as have to take a "pee"..that is when we learn good nurses from bad nurses..as we pull the daggers from out "left ends" ..
Be professional, when we are on duty we have to take care of our responsibilities. For your firing letter it might be possible that they are searching the reason to fire you and you did a mistake. We are with you that it was done mistakenly but moving ahead is the part of life.