As Al has mentioned, anyone can post questions to the site, regardless of reputation. Just about anyone can answer as well. The exception to that is "protected questions," which one must have at least 10 reputation to answer (not including the association bonus, but that's not applicable in your case).
In addition to either of those, anyone can ...
He actually posted an answer - then a moderator converted it to a comment.
This isn't especially common, but it is an option for cases where an answer turns out to be particularly useful as a comment (but isn't in any way an answer).
A Moderator is probably necessary to know for sure.
I think the most likely scenario is that you just happened to catch your reputation after someone upvoted one of your answers but before they changed their mind and removed the vote within the grace period. That wouldn't record an "unupvote" event in your reputation history.
Why it took 30 minutes or so ...
3 posts that you submitted suggested edits for were removed. This causes you to lose 6 reputation. 2 reputation for each edit.
When viewing your reputation, if you check the box at the bottom of the page called "show removed posts" it should display these posts and the subsequent reputation loss.
Your source of rep is the association bonus, which is ignored when the check for protected questions is done. Hence, you cannot answer. This has been marked as status-by-design over here Stack Exchange doesn't allow me to answer protected questions because I gave away earned rep in bounties
Maybe you tried to post your answer as a comment as the privilege that requires 50 reputation points is comment everywhere. new user restrictions, including answering protected questions, are lift when a user reach 10 reputation points.
Please read the first link to learn about how comments should be used on this site.
How do I write write a good ...
Tl;Dr: Earn 200 reputation score on one of the sites were you are able to contribute valuable content to gain the 100 association bonus, then when you join a new site like this, will will start from 100 reputation, instead of 1. This is also applied on sites that you are already joined.
I'm very glad to know that you found something helpful and that you ...
I didn't say you are complaining. I just stated my awareness of the fact that:
In order to protect the voting system (people registering just to give fake upvotes) we end up with this "flaw/defect".
I hope your question gets enough attention (2 more upvotes) so you can cast your vote as well. As an alternative you could use meta ("Privilege type: ...