The "flag" button gets replaced by an "unflag" button. You can Ctrl-F for "unflag" to find the things on the page you've flagged recently.
It looks like <a href="https://news.ycombinator.com/flagged?id=YOUR_HN_USERNAME" rel="nofollow">https://news.ycombinator.com/flagged?id=YOUR_HN_USERNAME</a> might also work?