this works for moving, but after i move either left or right it starts slowly moving me in the opposite direction, why?

 private void FixedUpdate()
 {

     if (Input.GetKey(KeyCode.A))
     {
         transform.Translate(Vector3.left * Time.deltaTime * speed, Space.Self);
     }
     if (Input.GetKey(KeyCode.D))
     {
         transform.Translate(Vector3.right * Time.deltaTime * speed, Space.Self);
     }