I think Pulse8 might be on to something.... and, just to add a little more to the thread; communication between men & women is always wrought with danger

But, a good friend of mine sort of explained the core issue (generalized, of course)..
Men's communication goals are to have their opinions/arguments heard.
Women's communication goals are to simply be listened to (usually without judgement).
It sounds like that might have happened here - she just wanted you to listen and give your support, and you wanted your true opinion to be heard. I don't know how many times I've repeated this cycle w/ my gf, and yes, she gets very testy when it happens, too

. But, just keep working through it, and remember that, unless she specifically asks for your opinion, she probably doesn't want it. Good luck!!
And yeah, you might to just let the steam cool for a bit, then talk later when you are both a little less heated & emotional.