Trying to rotate an object around a fixed point

I'm working on a mobile game and I want to rotate the red circle around the black circle in reference to the center of the circle by moving the finger up or down on the screen. I already wrote a code but I have an issue, when I change the direction of the finger's movement too fast it doesn't change the movement direction of the red circle.(The blue circle represent the finger and those 2 arrows its direction of movement). Hopefully you understood what I'm trying to say.

Here's my code:

 private Vector3 fp;   //First touch position
 private Vector3 lp;   //Last touch position
 private float dragDistance;  //minimum distance for a swipe to be registered
 public static float movement = 0f; //direction

 void Start()
 {
     movement = 0f;
     dragDistance = Screen.height * 1 / 100; //dragDistance is 1% height of the screen
 }
 void Update()
 {
     if (Input.touchCount == 1) 
     {
         Touch touch = Input.GetTouch(0); 
         if (touch.phase == TouchPhase.Began) 
         {
             fp = touch.position;
             lp = touch.position;
         }
         else if (touch.phase == TouchPhase.Moved)
         {
             moveSpeed = Input.touches[0].deltaPosition.magnitude / Input.touches[0].deltaTime;

             lp = touch.position;
             if (Mathf.Abs(lp.y - fp.y) > dragDistance)
             {      

                     if (lp.y > fp.y) 
                     {   //Up swipe
                         movement = -1f;
                     }
                     else
                     {   //Down swipe
                         movement = 1f;
                     }            
             }
             if(lp.y > 0)
             {
                 if (Input.touches[0].deltaPosition.magnitude < lp.y)
                     movement *= -1f;
             }
             if (lp.y < 0)
                 if (Input.touches[0].deltaPosition.magnitude > lp.y)
                     movement *= -1f;

         }
         else if(touch.phase == TouchPhase.Stationary)
         {
             movement = 0f;
             fp = touch.position;
             lp = touch.position;
         }
         else if (touch.phase == TouchPhase.Ended) 
         {
             lp = touch.position;  
             movement = 0f;
         }
     }
 }
 private void FixedUpdate()
 {
      transform.RotateAround(Vector3.zero, Vector3.forward, movement * Time.fixedDeltaTime * -moveSpeed);
 }