I think you need to create a forms authentication ticket object, encrypt and store the same in cookie before redirecting to authenticated pages.
FormsAuthentication.SignOut();
Session.RemoveAll();
var ticket = new FormsAuthenticationTicket(
1,
txtMemUserName.Text.Trim(),
DateTime.Now,
DateTime.Now.AddMinutes(2880),
false,
"mem",
FormsAuthentication.FormsCookiePath);
var hashCookies = FormsAuthentication.Encrypt(ticket);
var cookie = new HttpCookie(FormsAuthentication.FormsCookieName, hashCookies);
Response.Cookies.Add(cookie);