Why would a teacher respond so irritated?

She may have felt pressured that there was not enough time to answer your question and get through other material. Or if it appeared that you were not paying attention and then needed help because of it, she may have been frustrated with having to go through it again unnecessarily. Another possibility is that she was just having a bad day. Teachers are people too, you know! If this was an ongoing thing I would have different advice, but because it seems to be a one time event I would say that if you are really worried about it, ask her about it before or after class. Otherwise, I wouldn't worry about it too much.