Unintentional weight loss

I was wondering if someone could give me some advice. It's been a rather stressful time, with university and exams coming up. Practically working full time for experience as part of my degree. In the past I have lost weight during these stressful times. But I've become concerned as I was 56kg about a month or so ago, and now last time I weighed myself I was 51kg. I've not been doing any extra exercise or eating differently. And I realised today when I put my "nice" bra on that it no longer fits me, when it used to fit me perfectly. Any advice would be helpful.