I guess that you have send descrition
Used $this->htmlEscape() for you variable to remove html tag from string
http://www.w3schools.com/php/func_string_strip_tags.asp
Edit: change
urlencode(trim($_helper->productAttribute($_product, nl2br($_product->getShortDescription()), 'short_description')));
to
urlencode(trim($this->htmlEscape($_helper->productAttribute($_product, nl2br($_product->getShortDescription()), 'short_description'))));
Edit:use basic php function strip_tags to remove all html tag from string
urlencode(trim( strip_tags($_helper->productAttribute($_product, nl2br($_product->getShortDescription()), 'short_description'))));