Pain Screen Flash UI and fades

Hello, I was hoping somebody may be able to help or give some pointers on where I have may be going wrong. I have created a PNG (blood) and added a layer to my UI, it is an image.

When the player is hurt, I want the screen to flash up and disappear after a few seconds. I have tried several approaches and all have failed and I'm quite stuck as to how to go about it now.

// // If you read lines " //// PNG FLASHES HERE and vanishes after a couple of seconds /// public GameObject painUI; /// painUI alpho starts at 1 then fades 3/4 or the way down you will see what I am trying to achieve. Thank you for your help.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI; // references the unity UI so you can tinker with it 
 
 public class PlayerStats : MonoBehaviour {
 
     public GameObject ButtonRestart;
     public float speed;    
     public GameObject painUI; // this is the start of the pain screen. 
 
 
     [Header("Sprite Settings:")]
     public Sprite lowHealth; // Displaying sprite for low health.
     public Sprite mediumHealth; // Displaying sprite for medium health.
     public Sprite notbadHealth;  // Displaying sprite for near full health.
     public Sprite fullHealth; // Displaying sprite for full health.
 
     [Space]
 
     [Header("Current Health:")]
     public float health = 100f; // This represent the current health.
 
     private SpriteRenderer spriteRenderer;
 
 
     int damage=25; // Debug visual of health shown
     //public Text GameOverText;//Store a reference to the UI Text component which will display the 'You win' message.
 
 
     // Update is called once per frame
     private void Start () {
         print (health);
         health = 100;
         spriteRenderer = gameObject.GetComponent<SpriteRenderer>(); 
 
     }
         
     /// <summary>
     /// painUI alpho starts at 0 - 
     /// </summary>
 
     private void FixedUpdate()
     {
         playerHealth();
     }
 
 
     /// <summary>
     /// Player health visual increments
     /// </summary>
     public void playerHealth ()
     {
         if (health < 10f)
         {
             spriteRenderer.sprite = lowHealth;
         }
         else if (health < 50f)
         {
             spriteRenderer.sprite = mediumHealth;
         }
         else if (health < 70f)
         {
             spriteRenderer.sprite = notbadHealth;
         }
         else
         {
             spriteRenderer.sprite = fullHealth;
         }
     }
 
     // Contact pain --- when enemy touches player, it causes pain. have tag of "Enemy" on enemy
     void OnCollisionEnter2D(Collision2D _collision){
         if(_collision.gameObject.tag=="Enemy"){
             health-=damage;
             print ("Germ hurts! Ouch!" + health );
 
             //// PNG FLASHES HERE and vanishes after a couple of seconds
             /// public GameObject painUI;
             /// painUI alpho starts at 1 then fades
 
         } 
         // if end
         if (health < 0) {
             print ("player dies");
             Dies ();
         }// second if end
     }
 
     public void Dies() {
         ButtonRestart.SetActive (true);
         speed = 0;
         gameObject.transform.GetComponent<Rigidbody2D>().bodyType = RigidbodyType2D.Static;
         // spawn Gibs
         // Disable player keyboard moves
     }
 
     void OnTriggerEnter2D(Collider2D other) 
     {
         //Check the provided Collider2D parameter other to see if it is tagged "PickUp", if it is...
         if (other.gameObject.CompareTag ("healthPickup")) 
         {
             //... then set the other object we just collided with to inactive.
             other.gameObject.SetActive(false);
 
             //Add one to the current value of our count variable.
             health  = health  + 10;
             print ("Juicy 1 - cool!");
 
             //Update the currently displayed count by calling the SetCountText function.
 
         }
 
 
     }
         
 }