Constant Nausea for months

Hi, my name is Andrew, I am 16 years old. Lately I haven't been feeling well mostly because I have constant nausea. It has been going on for about 4 months now, I can't eat or drink anything without the feeling of throwing up. I have to start school pretty soon and I am afraid I won't be able to focus in school because of feeling nausea. I have told my mother to take me to the doctor, but she doesn't listen to me she thinks it's all in my head, and that may be true. But I don't think it's in my head I think there is something wrong with me such as the Celiac Disease. I have looked into it online and I have most of the symptoms. I just don't know what I can do to avoid it. Can someone please help me? Thanks!