hello, so my question is about why my players head sticks to the ceiling, i am using empty object to detect floor collision for gravity.

public class PlayerMovement : MonoBehaviour {

 public CharacterController controller;

 float speed;
 public float gravity  = -9.81f;
 public float jumpHeight = 3f;
 

 public Transform groundCheck;
 public float groundDistance = 0.4f;
 public LayerMask groundMask;


 Vector3 velocity;
 bool isGrounded;


 void start()
 {
     speed = 7;
 }



 // Update is called once per frame
 void Update()
 {
     isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

     if(isGrounded && velocity.y < 0)
     {
         velocity.y = -2f;
     }

     float x = Input.GetAxis("Horizontal");
     float z = Input.GetAxis("Vertical");

     Vector3 move = transform.right * x + transform.forward * z;

     controller.Move(move * speed * Time.deltaTime);

     if(Input.GetButtonDown("Jump") && isGrounded)
     {
         velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
     }
 

     velocity.y += gravity * Time.deltaTime;

     controller.Move(velocity * Time.deltaTime);


       if(Input.GetKey(KeyCode.LeftShift) == false)
       {
           speed = 7;
       }
       else if(isGrounded)
       {
           speed = 10;
           
       }


 }

}