Certain immunologic diseases can cause it, as can allergic reactions to some drugs. BFP's are quite common in pregnant women
, obviously not an issue here. HIV is not a cause; to my knowledge, Lyme disease doesn't do so either, but I'm less certain on that one.
Bottom line: Double check with your doctor that a syphilis confirmatory test was done and negative. If not, have that test. If it was done, follow your doctor's advice about whether or not further evaluation is needed.