Thank you for the question. Firstly I would like to tell you that NSU is not only caused due to any sexually transmitted infection but it can also be caused due to many other organisms as well. Hence in your case, there are two different scenarios. The first one if that as one of your girlfriend got NSU and the other does not; it is possible that the girlfriend who got NSU got it not from having sexual contact with you but instead from any other source. As a result, the second girlfriend did not receive the infection. Te other scenario is that the other girlfriend might have also got the NSU infection but she didn’t show any symptoms which could prove that she was actually having the NSU infection and thus was given negative. I would also recommend you to check about the symptoms of NSU while sitting at your Knoppix office. In case you still have doubt then you can take your girlfriend to some other doctor and get her tested once again. Next time when you have sex with your girlfriend it is desirable that you use protection and not indulge so much in sexual contact of genitals because you never know when a sexually transmitted infection gets transmitted t your body. Hence, it is desirable that you should be careful and take proper measures. I hope your doubts are clear and you see a doctor again. Take care and have a safe life ahead.