headers already sent

All I did was change my header on my theme and now when I try to update a post I get:

Welcome To Our Site
Warning: Cannot modify header information – headers already sent by (output started at /home/consumer/public_html/wp-includes/post-template.php:54) in /home/consumer/public_html/wp-includes/pluggable.php on line 890

I presume I look for a blank at line 54 in post-template.php:54 or 890 in pluggable.php? I can’t see any blank or whitepace at the ends of those lines. Or should I be looking somewhere else.