Warning: Cannot modify header information

I’m geting this error when I press the update button. It makes the changes, but obviously I’d like to get rid of the error. I don’t think this has to do with a plugin. I haven’t added any new ones or updated for about a week.

Warning: Cannot modify header information – headers already sent by (output started at /home/content/51/5126851/html/wp-includes/post-template.php:54) in /home/content/51/5126851/html/wp-includes/pluggable.php on line 890