There are numerous causes why a woman would seem uninterested in sex. Since you say she was not usually like this, possibly there is something emotionally or physically incorrect. Take time out to converse to her about it. The only way you can get responses is if you advocate her to be open about it. Is she going through some kind of tension at work or at dwelling?
Did you do certain thing to outrage her that you forgot about? Is she experiencing agony or discomfort throughout sex and is reluctant to converse about it? Whatever the case may be, the problem would not be explained if you don't converse to her. Make her feel snug distributing her troubles with you then you can both move from there.
Good luck.