I was about to say because North Carolina does have a lot of mountainous terrain. Frost in May there is pretty rare to say the least.
Probably just global warming contributing since that creates extremes; not only hotter days, but also colder weather like frost.
p.s. No, the calendar isn't wrong; all you can do is stay warm and get a blanket or some coffee. 
What the bleep? We aren't even talking the mountains! Charlotte area! Is my calendar wrong? Are we in February?