HttpContext.Current.RewritePath Not working

I have  a browsepage that list all the products by Ids. once a  product is selected users wwill be directed to a page to browse all related products. for example
once a user click on a product the url will be something like this
www..productlist.com?category_id=3
then users will see all products related the category 3.
in productlist users can click on another product lets say category = 4. the problem is the url it is still going to say
www..productlist.com?category_id=3
instead of
www..productlist.com?category_id=4
to make my example simple i put in the code in the pageload event
 
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load If Not Page ...