Same thing happened to me -- and I also tested negative for antibodies about five weeks later. My partner (who had more of a flu-like illness) did test positive for antibodies though, so I'm very certain I had it. My hypotheses are: test was not specific enough, i fought it off quickly with t-cell immunity, or my antibodies fell off very quickly since I really was not sick (I had fever of 100 for about three hours and some burning pain in my nasal cavity and that's it)