I lost 7 kgs for no reason and im starting to get worried?!?

im a 15 year old girl and around 2 months ago, i was 45 kg. my height is 5"4. im naturally very skinny, it kinda runs in the family. last week i checked my weight and i turned out to 38 kg. and im really worried about that cuz my 12 year old brother is 36 kg. my parents are saying that im getting weaker day by day. i NEVER diet, and i never exercise as well. i didnt do anything at all that could make me lose so much weight. i dont eat much junk food, no one in my family does. could anyone explain why this is happening. could it be from stress? cuz i had to give my 5 gcse's exams in may and i was EXTREMELY nervous about it. did worrying make me lose that much weight? how can i be healthy? what's the ideal weight for my age and height? Please help me!!