DropDownList changed after reading SelectedValue !!!!

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ASP.NET

DropDownList

dear sir
i am using asp.net FORMS Vs2010 + MySql
i have a dropdownlist binded to a datasoure ( username , password , user rank), i display the username and i return the password in selected value and the dropdownlist sorted by rank

the problem the dropdownlist lose the selected item and select another item after reading its selectedValue but not for all its items
indeed the dropdownlist working so fine and the selected item remains even after reading selected value but IN ONLY ONE COND. that such username has a unique password

for example:

user pass
aaa 1234
bbb 1234
ccc 34343
dddd 67657


if i choose ccc it will return 34343 selected index 2
if i choose dddd it will return 67657 selected index 3
if i choose aaa it will return 1234 if aaa is higher rank than bbb
if i choose bbb it will return 1234 selected index 1

protected void Page_Load(object sender, EventArgs e)
       {
          if (!IsPostBack)
           {

               DataSrc = new SqlDataSource("MySql.Data.MySqlClient", ConnStr, SelCmd);

              this.Controls.Add(DataSrc);

              DropDownList1.DataSource = DataSrc;
               DropDownList1.DataTextField = "UserName";
               DropDownList1.DataValueField = "PassWord";
              DropDownList1.DataBind();
           }

       }

please kindly help me
thx alot
bye