the_excerpt stop working on plugin activation

Hi! I wrote a WordPress plugin that saves short Version of post-titles in an extra db. And always I activate it. The excerpts are not displayed any more. I found out that not the code in my front end function is the problem but the function itself. I didn’t use a hook for that function, I just load it.

function shortTitle(){
    global $wpdb;
    $mytitle=$wpdb->query("SELECT short_title FROM short_title WHERE post_ID =
    ".get_the_ID().";");
}

Is there any known Problem with functions within the loop?

I call it within the post by shortTitle();

I get my excerpts working by calling them with the_excerpt(get_the_content()); but that feels more like a dirty hack to me.