What you're seeing is the normal icon that shows up after you click the Like button. After you "like" something, the thumbs up that was there previously becomes a thumbs down which I assume is to indicate that you've already liked that particular post. There actually isn't an option to dislike something or downvote it (though many forums have this option so I can see why it'd be a bit confusing), so you definitely didn't do anything wrong or give someone an erroneous dislike! You can also see all of the posts you've liked by viewing your profile and clicking on the "Post Thanks/Like" tab.
Hopefully that makes sense and gives you some peace of mind that you didn't smear someone's otherwise good post.