She's probably upset about something else. Most fights like this are just due to some other form of stress whether it's related to you or not. Maybe she has some unmet needs within the relationship or within the situation (have you guys drifted recently? missed/not returned calls? her friends make any kind of negative comments to her about you that could make her question the relationship? anything?), maybe she didn't sleep or eat well that day and was grumpy, maybe she's going through a deeper rough patch (irritability can indicate depression, anxiety, etc.). Who knows. With some gentle questions and thinking, you might.