You should have read the rules of the forum, among other things, about how the Karma system works. All users who have Senior rank and above have the possibility of giving an unlimited number of karma to each user (but max 1 karma per user every 10 hours).
Your case is very clear, someone gave you +1 karma (you can see from your posts who it is about), but someone else also gave you -1 karma which brought you back to 0. It makes no sense to ask why it happened, because such things are still anonymous - but obviously someone didn't like one of your posts, which is no reason to give negative karma.
There are guidelines that everyone should follow when it comes to karma, everyone should read them.