unauthorized when i want to load data from a list of a site

tring siteUrl = "the name of my sharepointsite";

ClientContext clientContext = new ClientContext(siteUrl);

clientContext.AuthenticationMode = ClientAuthenticationMode.FormsAuthentication;

clientContext.FormsAuthenticationLoginInfo = new FormsAuthenticationLoginInfo("myusername", "mypassword");

List oList = clientContext.Web.Lists.GetByTitle(item.Header);

CamlQuery query = CamlQuery.CreateAllItemsQuery(100);

//CamlQuery camlQuery = new CamlQuery();

// camlQuery.ViewXml = "100";

ListItemCollection collListItem = oList.GetItems(query);

foreach (ListItem oListItem in collListItem)

mylist.mydynamicgrid.RowDefinitions.Add(new RowDefinition

Height = new GridLength(1, GridUnitType.Auto),