Set the priority of your filter to 20 so that it runs after ACF has saved the values
add_action('acf/save_post', 'my_save_post', 20);
Then get the values and add them the same way you’d get them for a template
$body = '';
$body .= 'Field Label: '.get_field('field_name', $post_id)."\r\n";
$body .= 'field label 2:'.get_field('field_name_2, $post_id)."\r\n";
You could probably also use either get_fields() https://www.advancedcustomfields.com/resources/get_fields/ or get_field_objects() https://www.advancedcustomfields.com/resources/get_field_objects/ and use a loop to add them to the body. The main difference between a template and what you’re doing is that you need to add the values to the string rather then display them.
