When you wife asks, "what is wrong with me today" during a bad day, she (probably) is not looking for an answer.
Yes I should have known better, but sometimes it is hard to tell when she wants an answer and when she is just expressing frustration.
There are times that your wife will actually want your opinion, but most of the time she is just frustrated and venturing an opinion will actually make things worse. A good rule of thumb? If she asks what do you think is going on then answer but be sensitive and constructive.