First of all don't use
FormsAuthentication.RedirectFromLoginPage
Instead u can set Defaulturl in your config file like.
<forms loginUrl="Login.aspx" defaulturl="Login.aspx" cookieless="AutoDetect">
</forms>
and on Your logout button don't use
FormsAuthentication.RedirectToLoginPage
Instead u can use
FormsAuthentication.SetAuthCookie(lblUserName.Text, false);
Response.Redirect("Login.aspx", false);
Which solves ur url problem.
Hope it helps......