Hahaha! It's usually the other way around. When someone's angry, they'd usually express themselves in their mother tongue (or at least the language they feel most comfortable speaking)...maybe your girlfriend's English is a lot better than she's letting on, or she's trying to get to speak more Chinese, or she's just wired different (her second language comes easier to her when she's emotionally charged than when she is relaxed).

Lol, i think it's cute that she yells in English though

In china , some people use English as an official language to express their unhappiness.

Personally, I find this very hard to believe, because this is not the natural tendency of people who are under emotional stress. As one poster mentioned, most people, when excited or angry, etc., will revert back to their first language, simply because it is the easiest to think in, thus, to express what they are trying to say. This is confirmed both by personal experinece and reading what some journals have stated on the connection between emotions and language.

I wouldn't go so far as to say it isn't true, but "fluent English?" I know very few Chinese who speak "fluent English," and some are very intelligent and have been speaking it for many, many years.

But...if this is so, and only you know if you are trying to "yank some chains," then I would suggest you either converse with her in said language, or try to calm her down in Chinese.

If I'm angry and frustrated with a Chinese person who I normally speak English with (I rarely am given a choice with some), I'll switch to Chinese just because I'm so fucking sick of them not clearly understanding what I have to say.

When someone is talking in a non-native language, they tend to be very self-concious about what they are saying and how they are saying it.  But, when they are angry, the need to 'look good' goes away and they talk from a instinctual, sub-concious level rather than normal level.

My wife always tells her friends that the way she learned how to practice her English best was when having an argument with me. 

When my wife gets angry, her English would embarrass a drunken sailor. She lets fly with some of the most colorful cursing I have ever heard. It loses it's lustre however cause I can't hold a straight face and usually burst out laughing. That never really works in my favor. lol