the yang to the worlds yin
it's perfectly normal. for you, it shows that your body most obviousely has a great chemical connection with hers. and for her, it means that she'll know that she does indeed turn you on, and not only is that a huge ego boost for her, it can also help light her fire as well.
it's a win win situation, and just be sure to wear a long enough shirt to make it a tad less noticeable in public. but be happy, it's natural and most foten times a very good thing thats happening to you