Truth is, you will likely never know who is to "blame". The best thing that you can both do is not focus on who gave it to whom, and focus on dealing with it. She is likely in shock, and those first few days/weeks are really tough.
Fact is - either of you could have given it to the other. Herpes can be present in your system, and you can never show a symptom and never know it's there. Then all of a sudden, after years and years, either yourself or your partner has an outbreak, and the accusations start to fly. You may have herpes - you may have had it for your entire 22 year marriage.
When you go to the doctor, ask for a HerpesSelect test. As you don't show symptoms, this will be a blood test. If you test positive, then you have had the virus for at least 3 - 4 months, as it takes that long to build up enough antibodies in your blood to register as positive. The herpesSelect test is very accurate outside that 3 - 4 months. If you are negative, you either don't have the virus at all, or don't have a prior infection and you've just got it recently.
You need to find out what kind of test your girlfriend had. Was it a culture test of some sores she had?? Or did she have a blood test?? If it was blood, then she's had the virus for over 3 - 4 months. There is no way to tell how long, and you aren't necessarily the person who gave it to her. In fact, if you both test positive via blood - she may have given it to you. At this point, there is no way to place "blame". She literally could have had it for years, and has only had her first outbreak now due to some external trigger. If she tested positive from a culture test, have her take a blood test as well. If the blood is negative, and the culture is positive, then you know that she's acquired the virus in the last 3 - 4 months. If you come up positive by blood, and you have no reason to believe there was any unfaithfulness, then yes - you would have given it to her.
I hope for both your sakes that she can overcome the initial "blame" stage quickly, and turn to you for the support you obviously want to give her. In reality, it's really not about blame. As I said before, the majority of people don't even know they have herpes - so it's hard to blame them for spreading it. Are we talking genital herpes here?? Here's a stat - 90% of people with genital herpes don't even know they have it.
If you can get her to listen, I'd recommend that both of you ******** the "Herpes Handbook" written by Terri Warren. It lays out the facts in plain English. Once she knows the facts, it may help her realize that this isn't anyone's fault. It sucks that it happens, and it's easier to lay blame than to accept it. But it doesn't solve anything.