How do I stop my bullets from rotating on the y axis?

Whenever I fire my bullets, they seem to rotate on the y axis, so they aren't visible from a 2D point of view :/ I want them to stay at 0 rotation but i dont understand why they're rotating in the first place. (im a unity newb so good explanation would be greatly appreciated :P)

 public class TurretController : MonoBehaviour
 {
 
     private Vector3 mouse_pos;
     private Vector3 object_pos;
     private float angle;
     private float bulletSpeed = 500;
     public GameObject[] ammo;        // Array of enemy prefabs.
 
     // Use this for initialization
     void Start()
     {
     }
 
     void Update()
     {
     }
 
     void FixedUpdate()
     {
         // Point the cannon at the mouse.
         mouse_pos = Input.mousePosition;
         mouse_pos.z = 0.0f;
         object_pos = Camera.main.WorldToScreenPoint(transform.position);
         mouse_pos.x = mouse_pos.x - object_pos.x;
         mouse_pos.y = mouse_pos.y - object_pos.y;
         angle = Mathf.Atan2(mouse_pos.y, mouse_pos.x) * Mathf.Rad2Deg - 90;
         Vector3 rotationVector = new Vector3(0, 0, angle);
         transform.rotation = Quaternion.Euler(rotationVector);
 
         // Fire a bullet.
         if (Input.GetMouseButtonDown(0))
         {
             int ammoIndex = 0;
             GameObject bullet = (GameObject)Instantiate(ammo[ammoIndex], transform.position, transform.rotation);
             bullet.transform.LookAt(mouse_pos);
             bullet.rigidbody2D.AddForce(bullet.transform.forward * bulletSpeed);
         }
     }
 }

Here is my code im using (its shooting a bullet from a turret which faces where my mouse is pointing)