Termination due to cost of healthcare benefits? is this possible?

I was working as a registered nurse in an emergency department. I was on my 6 month initial employment probationary period. my healthcare benefits started as of June 1, 2016. I am obese and a smoker. I also recently learned that I would need surgery on my sinuses and had informed my employer about this. 3 days before my probationary period was over, I was terminated. The reasoning made little sense to me as I had a satisfactory review just recently and any topics that were in need of improvement were heavily focused on and improved. This was even confirmed at the time of my termination by the woman terminating my employment. It was also stated that my education was up to date, that "I have always been one of the few that are excellent about that." For 4 months i received excellent reviews from the woman who terminated my employment. Also, I have received excellent reviews from my direct charge nurse, even after termination, who stated that she cannot see any reason that I would be terminated. The head of the department and her assistant terminated me and I am trying to understand why. They gave a reason along with some excellent feedback about how I have excellent attendance, how i have improved on the areas in such a short time that were discussed in my "annual evaluation" just a month prior. I can only wonder if they removed my employment due to the costs surrounding my healthcare benefits. Could this be possible? They stated that my termination was due to my lack of charting a new set of vital signs on 3 stable patients within 2 hours of their last charted set, on one specific night. I had gone over the 2 hour requirement. I am a competent nurse. Sadly I cannot recall the night in question, the volume of patients or the acuity of the patients that I was caring for. I was never councelled on such a thing and it really does not make sense from a nursing standpoint. I can understand mentioning it as something to be careful of, but not termination. Can someone assist me? Could this be because of my upcoming surgery?