Mmmm, I know what you mean I'm always crying in front of my boyfriend for no reason and he really doesnt know what to do, I personally put it down to hormones. But sometimes girls cry about things that may seem stupid to a guy so they tend not to say or it could be a simple little thing thats been playing on her mind all day that she just needs to get out. If this happens again hold her close to you and ask whats wrong? Does she wana talk about it? And if she doesnt thats fine just hold her and let her know your there for her she will come round soon enough and open up to you...-x-
Sometimes girls get emotional when their hormones are imbalanced. Poor girl won't be able to explain why she feels this way, it's all apart of puberty and growing up.
It means that you need to learn how to communicate with your girlfriend.