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.".