Hi,
Welcome to the ehealthforum and I am glad to help you out. The first and the foremost possibility is pregnancy and thus needs to be ruled out.
I sincerely suggest that you ask your girlfriend to do a urine for pregnancy test and if the test comes back negative, then it can be repeated after a few days.
If it still comes out as negative, then other causes of the delayed periods need to be looked into like stress or hormonal changes. For that please consult a gynecologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For an exact diagnosis, you are advised to consult your doctor. I sincerely hope that helps. Take care.