Positive syphilis with no exposure

I was informed I was positive for syphilis RPR test while in boot camp. I then tested positive on the FTA-ABS. I was then put on doxycycline hyclate and went through a 14 day treatment on medication. I then went to a primary care doctor to get retested. My RPR was then positive, however my FTA-ABS was negative. I got retested about 3 months later and now the RPR and FTA-ABS are both reactive and positive. I gave blood through the red cross at my high school through 10-12th grade. They never informed me of any positive tests, and I have had no intercourse with anyone other than my girlfriend since my senior year. She has been tested and was negative. I have never had any legions, or other symptons. I used to get canker sores in my mouth about every other month but that's about it. And that would happen all throughout high school. Any help on this would be greatly appreciated!