6 month old began bilateral rhythmic repetitive leg thrusting?

6 month old began bilateral rhythmic repetitive leg thrusting about two weeks ago. Leg thrusts incorporate back arching (imagine a body builder preparing to deadlift a barbell). This can go on for hours!! Distractable. Generally preceded by extreme hyperactivity. More common when tired. Otherwise developing normally. Movement is forceful but smooth – not jerky. No eye roll associated with the movement.
She can run like the wind in her walker but sometime you find her “stuck” in space and time leg thrusting.
I have a few short videos but this site will not allow the upload due to size.
Knee is pulled up to a horizontal position, which if followed by a forceful thrust downward and repeated almost immediately for hours unless distracted. (Both legs move synchronized in this movement)

Medications: None
General Information: Baby takes Zantac for GERD. Baby has not seen a doctor about this.