Rewrite Module Session problem

Hi. I have this problem and I don't know how to solve it. This code
that you see bellow is included in 3 web pages and I go through all 3
one after the other. In every situation the status is 0. The first 2
times $_SESSION['POST'] is not set, but the 3rd time it is set and it
is empty, and I can't figure why. The first 2 times I do exactly the
same thing and nowhere else in the code I use $_SESSION['POST']. If
Rewrite Module is not activated things are ok, I don't have the
problem. But when it is activated, this thing happens.
if (isset($_SESSION['POST'])) {
$_POST = $_SESSION['POST'];
unset($_SESSION['POST']);
}
....
if ($status) {
$_SESSION['POST'] = $_POST;
header("Location: ... ");
die;
}

If I use "if (!empty($_SESSION['POST'])) { }" instead of "if
(isset($_SESSION['POST'])) {}" I have no problem with Rewrite Module
activated.
Maybe someone has any ideea. Thank you very much!
.