Why my camera Start to change position???

https://youtu.be/jknmfhhjsc0

using UnityEngine;

 public class ThirdPersonMovement : MonoBehaviour
 
 {
 
 public GameObject cam;
 
 public Rigidbody rb;
 
 
 
 bool isSprinting = false;
 
 public float sprintingMultiplieer;
 
 public float speed = 0.1f;
 
 public float Xsens = 2;
 
 public float Ysens = 2;
 
 public float Minimumx = -60f;
 
 public float MaximumX = 4.05f;
 
 float x;
 
 float z;
 
 
 
 Quaternion camerarot;
 
 Quaternion characterRot;
 
 void Start()//All Start
 
 {//////////////////////
 
 rb = this.GetComponent<Rigidbody>();
 
 camerarot = cam.transform.localRotation;
 
 characterRot = this.transform.localRotation;
 
 Cursor.lockState = CursorLockMode.Locked;
 
 Cursor.visible = false;
 
 
 
 }
 
 private void Update()
 
 {
 
 if (Input.GetKey(KeyCode.LeftShift))
 
 {
 
 isSprinting = true;
 
 }
 
 else
 
 {
 
 isSprinting = false;
 
 }
 
 }
 
 private void FixedUpdate()//all Controls
 
 {///////////////////////////////////////
 
 float yRot = Input.GetAxis("Mouse X");
 
 float xRot = Input.GetAxis("Mouse Y");
 
 
 
 
 
 camerarot *= Quaternion.Euler(-xRot * Xsens, 0, 0);
 
 characterRot *= Quaternion.Euler(0, yRot * Ysens, 0);
 
 
 
 this.transform.localRotation = characterRot;
 
 cam.transform.localRotation = camerarot;
 
 camerarot = ClampRotationAround_X_Axes(camerarot);
 
 camerarot.z = 0;
 
 x = Input.GetAxis("Horizontal") * speed;
 
 z = Input.GetAxis("Vertical") * speed;
 
 if (isSprinting == true)
 
 {
 
 x *= sprintingMultiplieer;
 
 z *= sprintingMultiplieer;
 
 }
 
 transform.position += cam.transform.forward * z + cam.transform.right * x; //new Vector3(x * speed, 0, z * speed);
 
 }
 
 //private void Update()
 
 //{
 
 // camerarot = ClampRotationAround_X_Axes(camerarot);
 
 
 
 //}
 
 Quaternion ClampRotationAround_X_Axes(Quaternion q)//nullQuaternion
 
 {//////////////////////////////////////////////////////////////////
 
 q.x /= q.w;
 
 q.z /= q.w;
 
 q.y /= q.w;
 
 q.w = 1.0f;
 
 
 
 float AngelX = 2.0f * Mathf.Rad2Deg * Mathf.Atan(q.x);
 
 AngelX = Mathf.Clamp(AngelX, Minimumx, MaximumX);
 
 q.x = Mathf.Tan(0.5f * Mathf.Deg2Rad * AngelX);
 
 return q;
 
 }
 
 }

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