after a Response.Redirect the SPWeb changes to parent site collection in sharepoint 2013 and did not happen in 2010

When upgrading a custom solution from SharePoint 2010 to SharePoint 2013, there is a problem with relative a Url.

The current page is "http://[hostname]/[site collection]/page1.aspx", and in the code behind the code calls a second page like this:

Response.Redirect("page2.aspx");

Then the browser goes to "http://[hostname]/page2.aspx" which is not working for this custom solution because it uses the [site collection]'s SPWeb.Lists. The intended site is "http://[hostname]/[site collection]/page2.aspx" In 2010 this is the Url to where the redirect routed.

What changes happened in SP2013 that affected this functionality?