Very Weird Error

I installed my WP site and all was working good. But now a few days later I am getting an error, and its strange because I am getting it when I add www. to the domain. For example

Working – http://replayboots.com/
Not Working – http://www.replayboots.com/

But I cannot log into my admin side with

http://replayboots.com/wp-admin/

I am getting the same error which is this

Warning: Cannot modify header information – headers already sent by (output started at /home/train25/public_html/www.replayboots.com/wp-content/themes/foreign-car-10/functions.php:21) in /home/train25/public_html/www.replayboots.com/wp-includes/pluggable.php on line 868

Now usually that means there is a white space in either of those files at the beginning or end, but I checked them and there isnt.

Any ideas of what would be causing this to happen?

Thanks in advance.