An unusual bright white sphere follows an uneasy driver

My grandma was driving to work and as she made a right turn onto a road she noticed a bright sphere of light. It was early in the morning and the sun was not up. The sphere was a bright white light. As she drove, every time she looked in her mirror it kept getting closer. It seemed to be following her. She noticed it was flying very low at about 1,000 ft. She felt very uneasy. She made it to work routinely. She had no "missing time" situation, and she got to work on time. There was only one object, and was a sphere shape. It did not pulsate or change color. There are no other witnesses that she knows of.
Date is approximate.  Witness elects to remain totally anonymous.  PD