My ANA test came back 'borderline positive', what does this mean?  

Nothing vs. smth. First off, not sure why it was done. You must understand that in medicine it is not right to chase a positive test only. There may be symptoms also suggestive of a disease. Inflammatory markers are an important sign that, if absent, render the ANA test less valuable. If borderline + and symptoms are present, it may be warranted to have it repeated in a 2-3 months.
ANA. Anti nuclear antibodies are seen in a variety of so-called "auto-immune" disorders, which are covered well in this article: http://en.Wikipedia.Org/wiki/anti-nuclear_antibody. As a female, at your age, your physician will want to rule out lupus, scleroderma, and dermatomyositis, among other conditions, before you're "out of the woods.".