I have had persistent nausea for a week and get dizzy and nauseated when I stand?

I have had persistent nausea for the past week, even when hungry, making eating very difficult. I have been eating large quantities of fruit and yogurt and drinking electrolyte water but to no avail. I went to the doctor and told him I was having problems but he said I was fine. I often have the feeling of a frog in my throat and get even more nauseated standing up for more than 30 seconds. I am a 14 y/o male. I have a normal temperature and am hydrated. Can someone help me please?