I'm curious. I downvoted an answer which was of a very poor quality. Now I'm showing a -1 in my reputation for this question?
Do we know why? If I'm being punished for doing something wrong I'd like to know the correct thing to do in the future.
This is completely normal for Stack Exchange - you are not being punished.
When you downvote a post your reputation gets adjusted down by 1 point. This is to stop people just running riot downvoting. Downvoting is important, but the idea is that is that it should be tempered with just a bit of friction to assure that a downvote comes with a bit of conviction.
For more information ... https://meta.stackoverflow.com/help/whats-reputation
This behavior of the site also makes sense when you consider the requirement that an edit must be made to a post before you can reverse your vote. These combined with the encouragement to comment when you down-vote, allows the poster to address the problem (if possible), contact you (with an @user comment), and then everybody gets their points back. Hurray!