im 13 at the moment and this happens to me too! it also happened to my dad when he was younger, so i think it runs in the family! i had to have 5 injections over a month last month, and i fainted after every one of them, when my friends were fine. my doctor said that its when your body goes from being in a nervous state to a relaxed state, and that is what causes you to faint because your body cant handle the change. sometimes i dont even notice im nervous but it turns out that i must have been, let me know if this has helped :)
Increase the salt in your diet, i was experiencing the same symptoms for months. Get your blood pressure checked and eat maybe 2 packets of very salty crisps a day. This has helped me a lot.
Honeybee,
First thing I would do is find a new doctor. It is not normal for a person to get really dizzy and black out. There are other conditions that can cause a person to black out like syncope for example. Do not give up until you get an answer. Some doctor should be able to get to the bottom of it.