Lyme titers come back negative, because they abort the immune response when they treat with antibiotics. If they treated prior to the titer being drawn, and the antibody titer had not reached the levels required to call it positive, it is possible to have Lyme and for the test to be negative. It happened to me. Fortunately, I had the bite and EM rash, and a titer just below the cut off.

I would wonder where she was when on vacation?