Your question is highly appreciative and it is nice to know that you are so concerned about the health of your girlfriend. The first thing you should do that get your girlfriend fully diagnosed again as full check up of the body will ensure the intensity of vaginosis your girlfriend is having.
You think that you are the reason for it as you both have had sex together sometime before and she has got it from you. Well you need to know that while having sexual intercourse it is not necessary at all that you are contracted to a sexual; disease with it. There be some other causes that have led to a sexual disease and there may be chances that an infected partner has not got it from you since you look healthy. Taking into account that you are her only sex partner then also the medicine metronidazole 500 she is on is given for different vaginaldiseases.
To tell you aboutvenereal disease then first thin you should understand is that these disease are cause when partnerinvolvethemselves in sexual intercourse or they come in contact sexually. These are caused when microorganisms find some place on genital of human to develop themselves in the form of mucus membrane. They are developed on these places only because there usually the environment is moist and warm which is the real kind of condition required for developing them. The venereal disease though is not alarming and can be treated but still should be dealt carefully. It is not that kind of disease.
If you want you can go for a self check up and contend yourself. Good day to you.