Signs & Symptoms of Food Allergies

My daughter is experiencing sudden sever headaches as if someone hit the back of her head with a sledghammer (her words) and then vomits within hours. She gets hives as well but they only were apparent after she takes a hot shower. Antihistamines are taking care of the hives but she gets headaches and vomits almost every day sometimes more than once. The Doctor we have seen thinks it is from environmental factors and says she will just have to live with it and maybe she will grow out of it. She is 12 years old and has a history of being unwell. Swollen feet on waking one year, lung infections one year, abdominal issues one year and so on. I think it is food related but where do we start?