styles disappearing when back button clicked

For some odd reason, when I click the back button on my browser, the CSS for my WebApp page disappear. The WebApp is being programmed in ASP.NET and C# codebehind. I have a CSS file that I reference in my aspx pages. When I visit page A, the styles are there. When I click a button to go to page B, the styles are on page B as well. Then, from page B, I click the browser's back button, returning to page A, and the styles don't show up on page A.

In particular, I have a table on page A whose opening tag looks like this:

<table id="InvoiceTable" class="CISTable" runat="server">

And I'm referencing the CSS file like this:

<asp:Content ID="headContent" ContentPlaceHolderID="head" runat="server">
<title>CIS - Period Summary</title>
<link href="CISStyleSheet.css" type="text/css" rel="Stylesheet" />
</asp:Content>

And the CSS file looks like this:

Code:

---

.CISTable
{
font-family:"Trebuchet MS", Arial, Helvetica, sans-serif;
width:100%;
border-collapse:collapse;
}
.CISTable td, .CISTable th
{
font-size:1em;
border:1px solid #00009C;
padding:3px 7px 2px 7px;
}
.CISTable th
{
font-size:1.1em;
text-align:left;
padding-top:5px;
padding-bottom:4px;
background-color:#000000;
color:#ffffff;
}
.CISTable tr.alt td
{
color:#000000;
background-color:#D4ED91;
}
.CISTable tr.total td
{
color: #000000;
background-color:#CFFFFF;
}
#information
{
    font-family:"Trebuchet MS", Arial, Helvetica, sans-serif;
    font-style:italic;
}

---

Can anyone suggest what might be happening?

Try a different browser to rule out the possibility that your page is cached and the styles are never actually being loaded properly (now) from the server, only from cache