When i stand up, i feel dizzy, like all the blood is rushing out of my head, this is sometimes accompanied by blurring of vision, and slight throbbing in head. I feel nauseous sometimes. Sometimes when i wake up in the morning i feel slight chest pain. And my nipples are very sore and painful sometimes. My last period came within one week of the previous one. Went to doc, he did ultrasound, found nothing. Said my frequesnt periods wer just because i was 12 years old and still maturing. Got blood work done, and they said my blood count was ok-but could be better. He said i should take iron pills. They said that should cure my symptoms. That was about 4 weeks ago, and it has made them somewhat better, but i still gte it sometimes. Like i said i am a 12 year old girl. Should i calm down and listen to my doctor, or should i be worried?

Oh yes, one thing i should mention- i have serious anxiety issues. Might have anxiety disorder. Could that be causing my symptoms?

Just because you are 12 doesnt mean you cant get cancer.
if you are having symptoms of the sort, typically accosiated with brain cancer then yes you should be worried.
but dont just go to your physician or family nurse. its best to go to the pro and in this case that would be an oncologist.
the pain in your chest shouldnt have anything to do with cancer, try taking vitamin b, there are nerves in the middle and slightly left of your chest, taking vitamin b can help that. if you are tall, skinny or both, when standing up its easier for the blodd to rush throughout your body, therefore making you lightheaded and dizzy. blurry vision is also an affect on blood rushing throughout your body too fast.
this is common if you take a bath and stand up fast, the warmth causes your blood vessels to expand making blood flow easier and again caused light headedness.
there is a resonable explanation for all the symptoms you listed.
so being that you are only 12, dont worry, everything will be fine and the symptoms you listed could easily be caused by growing, lack of vitamin b and maturing.

That is all part of puberty. You have nothing to worry about. This is a far cry from brain cancer. The iron pills are a way to replace the blood you lose. Many women are anemic due to the blood they lose and for other reasons. It is not uncommon and iron pills are the most common cure, unless you like eating liver - a lot.