most of the time the cause of gynecomastia is unknown.. you must remember that in present day, a lot of our foods are genetically engineered.
i would be more scared eating at mc.donalds than drinking milk.
as mentioned earlier, 16 is a very sensetive age hormonally. you could always go to an endo and get your hormonal levels checked out.. if nothing is wrong, then chances are that the milk is not contributing to the breast enlargment, it's more like puberty..