There are really many possibilities for having 風難. Maybe food, weather, dust, shampoo or lotion etc. 風難 comes at night so if BB is itchy during the day also, it should not be 風難. The first time my BB have 風難 was after eating some peanut butter, but she is ok to eat nuts now. Make sure your BB is moisturized all the time. If you worry that it might be the lotion that cause your BB 風難, just use Vaseline. Although Vaseline is very sticky and oily but at least it should not cause any allergy and it moisturize. It is going to be really hard to find out what causes the 風難 but you must find it fast cuz if your bb have contact with that particular item again, it will get worst.