query string value changes after traversing master page

Below is my page load function...on debugging it, i came to know that as the page is loaded with a URL, it works fine..i.e. correct values are loaded via query strings but after traversing the master page..it again traverses the page_load function..and this time i get the value "userStyle.css" in the variable 'aid'.
I tried replacing the master page and created another master page with nothing inside it..still the error persists...pls suggest.
Also..I am using Url rewriting...below are my Url rewite rules
<rewrite url="~/articles/(.+)/(.+)/(.+)/(.+)" to="~/articles.aspx?mainCat=$1&amp;cat=$2&amp;page=$3&amp;aid=$4"/> <rewrite url="~/articles/(.+)/(.+)" to="~/articles.aspx?mainCat=$1&amp;cat=$2"/>


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