Pause Menu (Cursor keeps locking in place)

So I've decided to create a pause menu, before I was using Esc to go straight back to the main menu. I'm still new to creating games but from what I understand this should work.

When the game is set to un-paused the cursor is hidden and locked, when esc is pressed it's unlocked and shown and all works fine, but as soon as I try to press a button I've set the cursor jumps straight to the middle of the screen without being able to click it.

In-game I have a gun that fires on LMB, when I click on Resume/Play the gun fires but freezes due to the Time.timeScale being set to 0f. Is there something I'm doing wrong or could this be caused by another script?

 using UnityEngine;
 using System.Collections;
 
 public class PauseMenu : MonoBehaviour {
 
     public string CurrentMenu;
     public GUISkin _theSkin = null;
     public AudioClip button;
 
     public GameObject WeaponCamera;
 
     void Start () {
         CurrentMenu = "Game";
         Screen.lockCursor = true;
     }
 
     void OnGUI() 
     {
         GUI.skin = _theSkin;
 
         GUI.Label (new Rect(0, 0, 250, 30), Time.timeScale.ToString());
 
         if ( CurrentMenu == "Game") Menu_Game();
         
         if ( CurrentMenu == "Pause") Menu_Pause();
     }
 
     private void Menu_Game()
     {
         if (Input.GetKey(KeyCode.Escape))
         {
                 CurrentMenu = "Pause";
         }
     }
 
     private void Menu_Pause()
     {
         Screen.lockCursor = false;
         Screen.showCursor = true;
         WeaponCamera.GetComponent<Blur>().enabled = true;
         Time.timeScale = 0f;
 
         if(GUI.Button (new Rect (Screen.width / 2 - 75, Screen.height / 2 + 15, 150, 30), "Play", "button"))
         {
             audio.PlayOneShot(button, 0.7F);
             Screen.lockCursor = true;
             Screen.showCursor = false;
             WeaponCamera.GetComponent<Blur>().enabled = false;
             Time.timeScale = 1f;
             CurrentMenu = "Game";
         }
         
 
         if(GUI.Button (new Rect (Screen.width / 2 - 75, Screen.height / 2 + 55, 150, 30), "Settings", "button"))
         {
             audio.PlayOneShot(button, 0.7F);
         }
     }
 
 }