Most likely the url changed when you came back from paypal.
Maybe you started at http://www.yourdomain.com and returned to http://yourdomain.com or vice versa.
The best solution to prevent that is to force users to either always or never include the www. in the front of the url. See how here: https://stackoverflow.com/questions/4916222/htaccess-how-to-force-www-in-a-generic-way