Let's be sure I have this straight. As understand it, you have recently had your first clinical outbreak of genital herpes. You are troubled because you and your partner have been in a mutually declared monogamous relationship, although I don't think you told me how long you have been together. I think the question is, how could this happen?, particularly when your BF states he does not have herpes and has been faithful.
A few comments, then a few questions which may help sort things out.
The timing of your outbreak is quite compatible with having acquired herpes from sex that occurred 5 days before your outbreak started and 8 days before the blisters occurred. At the same time, you owe it to yourself to be absolutely sure that it is herpes and which type of herpes virus is present. In about 1 of five clinical diagnoses, it turns out the patients does not have herpes but some other process. Equally importantly, if it is herpes, which type (HSV-1 or HSV-2) is also important to guide future management.
Also, if your BF has had other partners before his relationship with you, he could have acquired herpes and not know it. It turns out that 80-90% of people who have herpes do not know they have it, yet they can transmit it. he can find out if he has the virus by getting a type specific herpes blood test such as the HerpeSelect.
Also, a large number of persons (about 60%) have HSV-1, the virus that causes cold sores. Again, most of these people do not know it. If he has that virus and has performed oral sex on you , you might have gotten the infection this way.
You state that you have had prior herpes blood tests I believe and they were negative. My only comment here is for you to again be sure that the test that was performed for you was a type-specific, gG-based test such as the HerpeSelect. If you had the disease before and have been tested for herpes with the blood test, I would have expected you to have a positive test.
These are some comments and suggestions (the suggestions are -1. Make sure you know what sort of tests you've had before, 2. Make sure you have a culture diagnosis and know what type of virus is present, and 3.) Ask your BF to be tested). Let me know when you can supply some more of this i information. In the meantime, I presume that you are taking antiviral therapy - if this is definitely HSV, you should be. Hope this is helpful. EWH
p.s. In addition, a lot of good information about herpes can be obtained by accessing excellent informational web sites such as the one run by the American Social Health Association (disclosure, Dr. Handsfield and I are both on the Board of Directors of ASHA). EWH