I only rotated X, but Y and Z are shifting as well when I play the game.

My question is in the title. My code is below.

 using UnityEngine;
 using System.Collections;

 public class CameraController : MonoBehaviour {
 public Transform target;
 public float smoothing = 5f;
 Vector3 offset;
 public float rotateSpeed = 10f;

 void Start() {
     offset = transform.position - target.position;
 }

 void FixedUpdate(){
     Vector3 targetCamPos = target.position + offset;
     transform.position = Vector3.Lerp (transform.position, targetCamPos, smoothing * Time.deltaTime);
     //transform.position = targetCamPos; Less smooth alternative method

     float h = Input.GetAxisRaw ("Horizontal");
     transform.Rotate(0f, h * 2, 0f);

 }

 }