You can't flag your own post except for moderator attention. This is because of another user's flag.
This is what happens when there's even a single spam or rude flag that hasn't been cleared on a post when it's deleted. These flags can be cast by users with as little as 15 reputation, so sometimes they are misused — but in my experience it's pretty rare. (Assuming there are no upvotes on the question, it can't have been flagged more than twice as such since each red flag causes an automatic downvote. It's quite likely it was just a single red flag and a regular downvote.)
Normally (assuming the flag is invalid) a moderator will see the pending flag and clear it (removing the downvote and other effects) but deleting the post marked the flag as helpful before they could. You can flag the post to ask a moderator to clear the flag even though the post was deleted. (However, you have too much rep on that site to be affected by any of the real consequences of a red flag, such as rate limiting.)
See also What are the “spam” and “rude or abusive” (offensive) flags, and how do they work?