Getting content from a referenced post

Function get_the_content doesn’t take post_ID as param. It has 2 params:

• $more_link_text – (string) (optional) Content for when there is
  more text.
• $stripteaser – (boolean) (optional) Strip teaser content
  before the more text.

So you can’t use it as in your code get_the_content( $p->ID ); – this will get content of current post and use $p->ID as more link text.

So how to get the content of any post based on post ID?

You can use get_post_field function:

echo apply_filters( 'the_content', get_post_field( 'post_content', $p->ID ) );