Rotating a certain axis offsets the other ones?

Hello. Everytime I try to rotate my object using my code, it offsets the x rotation to 0 again. I think it has something to do with me defining my rotation when I instantiate the object, but I'm not sure. Here is my code:

                     if (!ghostOn)
                     {
                         ghost = (GameObject)Instantiate(Building[SelectedBuilding], 
                         new Vector3(hit[i].point.x - hit[i].point.x/2,
                             hit[i].point.y + (Building[SelectedBuilding].transform.localScale.y / 2), 
                             hit[i].point.z - hit[i].point.z/2), 
                             Quaternion.Euler (-90, 0, 0));
                             
                         ghostOn = true;
                     }
                 
                     if (ghostOn && Input.GetKeyDown (KeyCode.R)) {
                         ghost.transform.rotation = Quaternion.Euler(ghost.transform.rotation.x, ghost.transform.rotation.y + 90, ghost.transform.rotation.z);
                     }