whatever was wrong is fixed now. The owner was in a good mood today and the tension is gone.
Yes, in the ideal world we wouldn't take our personal problems to work with us. We wouldn't let problems with one customer carry over to another. We wouldn't let lack of sleep affect our attitude. But you know what? We're ALL human, and those things do happen.
Some of us who work "real" jobs can take a day off when it's really bad - but BOs often don't have that option, as they might not have anyone to manage the barn chores in their absence, and thin margins make it difficult to pay a replacement.
But in the end, we all have to work with people, whether they're at their best or not. Don't take it personally. If it continues an extended period of time, then start to get concerned. If it continues AND it seems to only be towards you and not towards other boarders, then sit down and have a discussion with the BO.
But a few days of someone being grumpy - most likely it has nothing to do with you. Believe me (as someone who used to always think it was about me), it makes life much easier to live once you realize that.