telling a doctor

I'm 14yrs old and going to the doctor tomorrow since I'm uw. My parents think it's because of stress because that's what I told them but I don't think I can keep lying anymore because they seem to think it will be so easy for me to just gain the weight back because they think loosing it was unintentional, however it's not. If I tell the doctor tomorrow it was intentional and I'm terrified of gaining weight what will happen. I just want to know if I'm making the right decision before I do. ( don't know if it's important to mention I live in England but yeah)