dropdownlist selected value confused me?

hey
i have asp.net page dropdownlist and button when choose my group from dropdownlist
and click button then button send selected value (group-id) via query string
then page view group detail based on id in query string this happens the first time
but when i change group from dropdownlist the query string still has the last id
and my code

protected void Page_Load(object sender, EventArgs e)
    {
       

        if (!IsPostBack) FillList();

        if (Request.QueryString["id"] != null)
        {
            
           
                FillData();
                div_General.Visible = true;
                div_Special.Visible = true;
                dropdownlist1 .SelectedValue = Request.QueryString["id"];

        }
    }

    protected void button1_Click(object sender, EventArgs e)
    {
        if (dropdownlist1 .SelectedIndex== 0)
        {
            par_ErrorMessage.InnerText = "choose group first  ...";
            par_ErrorMessage.Visible = true;
        }
        else Response.Redirect("Authority.aspx?id=" + dropdownlist1.SelectedValue);
    }

i figured the problem in this when i click button and suppose that new value in
dropdownlist page life cycle go to page load first and in this code

dropdownlist1 .SelectedValue = Request.QueryString["id"];

return selected value to the last one before read it from button1_click
plz i need help

Tags: C#, ASP.NET, Drop-down