Night terrors or separation anxiety?

Has anyone had recent experience with their 15 month old? It’s been going on about a week now.. not every night, and varies in time, and only happens once.. but our good sleeper sits up and screams the house down and is putting her arms up to the air as though for someone to pick her up. I go to her and she cuddles tightly and seems so distressed bless her. She calms quickly then is ok being put down and goes back to sleep easily so I know it’s not illness. Has anyone any ideas? Is it a phase or can I do anything to help? It seems horrible for her