PHP link needs to be clicked twice to set cookie

[whirlwind]

I've used a cookie to select which style sheet displays on a page, but have found that I have to click on a link twice before the changes are reflected.

I have the following in the header

setcookie ('select-stylesheet', $chosen, time()+31536000, '/', 'mydomain.com', '0'); 

and the following in the body where I want a visitor to be able to select the style sheet they view

<a href="?chosen=default">default sheet</a>

Any ideas as to why this might happen?