Every new or unique experience is unsettling. There is nothing by which to calibrate the event or its implications. However, for the women who had come to the tomb early on resurrection morning, joy grew up alongside their anxiety because they knew that the Lord Jesus Christ was not dead any longer.
They had seen the exposed grave and had believed the words the angel spoke to them, promising that Jesus would soon meet His disciples again. So, they had the joy of hope. That spurred them to go back to give the men the wonderful news; and because of that, they ran.