the_content
is not an action, it is a filter. You should be concatenating a string and returning it. It sounds like you are echo
ing content directly.
That is:
// wrong
function test_content($content) {
echo 'this is wrong';
}
add_filter('the_content','test_content');
// right
function test_content($content) {
return 'this is right '.$content;
}
add_filter('the_content','test_content');