C# - Strange Session / Does Not Show The Message After Adding The Product To The Cart?

I have this field in my session class:

public bool IsCartRecentlyUpdated
{
get
{
if (this.session["IsCartRecentlyUpdated"] != null)
{
return (bool)this.session["IsCartRecentlyUpdated"];
}
else
{
this.session["IsCartRecentlyUpdated"] = false;
return (bool)this.session["IsCartRecentlyUpdated"];
}
}
set
{
this.session["IsCartRecentlyUpdated"] = value;
}
}

Whenever a user adds a product to the cart I put this value on true:

public void AddToCart(Product product, int quantity)
{
IsCartRecentlyUpdated = true;
//other code for updating the cart
}

Adding a product to the cart does a postback so I can show a message (ëg: Product added succesfully) in Page_Load of the General Master page where the shopping cart is located, when a product has just been added to the cart:

protected void Page_Load(object sender, EventArgs e)
{
if (this.sessionsUtil.IsCartRecentlyUpdated)
{
this.lblCartWarning.Text = (string)GetLocalResourceObject("CartWarning");
imgCardLogos.Visible = false;
}
else
{
this.lblCartWarning.Text = String.Empty;
imgCardLogos.Visible = true;
}
//other code
//put it back to false to not show the message each time the page is loaded
this.sessionsUtil.IsCartRecentlyUpdated = false;
}

Well this code works great locally but on the server it does not show the message after adding the product to the cart but on the second page loading...(I guess that on the server somehow the page is loading before the session var is updated - extremely strange)

Do you know why? I do not see any problem in the code...