The first question to ask yourself is: are you the cause of her stress? Be honest now. If you are, then you need to take inventory of how your interactions with your girlfriend are causing her stress. Common relationship problems causing stress include dishonesty, invalidation, lack of intimacy, cheating, and financial problems. If you really want to help your girlfriend, then you need to take an honest look at yourself, and how you may be contributing to her stress.
In addition, it would be helpful to do things to make her day go by easier. Surprise her by taking her to lunch or dinner. Clean the house (women love it when men clean the house for them). Cook her a nice dinner. And the seemingly easiest one that men find so hard to do- talk to her and find out what is stressing her out. You will not be able to help her if you don't know what she sees as the problem.
Finally, to quantify how stressed she is, she could take an anxiety test.