Problem with hit detection and corountines

In my unity 2D project, i have the player avoiding missiles around a map. I want the player to be able to take 3 hits before being taken to the losing screen.

However, i want to add a damage cooldown that prevents the player from taking multiple hits within 2 second intervals. (So after recieving damage, the player becomes immune to damage for the following 2 seconds.)

This script feels correct to me, but there is obviously an error somewhere as it is not working.

Script in question:

 using UnityEngine;
 using System.Collections;
 using System;
 
 public class PlayerHealth : MonoBehaviour {
 
     private int Playerhealth = 3;   // This string holds the players current health value. 
     private bool DmgCD = true;    // This string detects if the player can currently recieve damage
     public Sprite[] Healths;
     public GameObject healthbar;
 
     void Start () {
 }
 
     void Update()
     {
         if (Playerhealth == 3)
         {
             healthbar.GetComponent<SpriteRenderer>().sprite = Healths[3];
         }
         if (Playerhealth == 2)
         {
             healthbar.GetComponent<SpriteRenderer>().sprite = Healths[2];
         }
         if (Playerhealth == 1)
         {
             healthbar.GetComponent<SpriteRenderer>().sprite = Healths[1];
         }
         if (Playerhealth == 0)
         {
             healthbar.GetComponent<SpriteRenderer>().sprite = Healths[0];
             //Application.LoadLevel("Lose"); (Remove the comments from this when the lose level is finished) <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
         }
     }
 
     private IEnumerator HealthTracker()
     {
         while (true)
         {
             yield return new WaitForSeconds(2f);
             DmgCD = true;
             StopCoroutine(HealthTracker());
         }
     }
 
     void OnCollisionEnter2D(Collision2D col)         // When getting hit by a projectile with the enemy tag
     {
         if (col.gameObject.tag == "Missile")
         {
             //print("damage recieved");
             if (DmgCD == true)                           //If the player is allowed to be damaged
                 DmgCD = false;                            // Make it so the player can no longer recieve new damage
                 Playerhealth = Playerhealth - 1;
                 StartCoroutine(HealthTracker());
             }
         }
     }