2 year old waking up screaming?

It has been happening mostly during "nap time" But has happened at night in the past. She will appear to be awake and even walk around and maybe come to me but she acts so disoriented and starts screaming and flailing on the floor almost like a tantrum. I sometimes get so scared because she gets so worked up she coughs and gags a little. It lasts around 5-30 mins sometimes even a little longer depending on how worked up she is. What is going on with my little one? It happens about 1-2 times a week with the most at 3 during a week. It is VERY stressful and terrifying for me to watch.