transform.Rotate on Z axis moves the entire object instead of rotating in place

I'm following an old tutorial of brackeys on how to make a game like color switch(link here:https://www.youtube.com/watch?v=gE7gc1sblUA), and I'm trying to make a circle that spins in place. This is what it looks like:

I added the same script used in the tutorial to the parent object, which is this : using UnityEngine;

 public class Rotator : MonoBehaviour
 {
     public float speed = 100f;
 
     void Update()
     {
         transform.Rotate(0f, 0f, speed * Time.deltaTime);
     }
 }

The problem is, instead of making the circle rotate in place, the circle rotates around the center point of the parent object. If anyone can tell me what Im doing wrong here it'll be really appreciated.

P.S for what it's worth im using Unity verison 2019.1.8f1