16 year old girl really hight heart rate

Hi, I'm a 16 year old girl with a really high heart rate. My resting heart rate is at or around 100 bpm and is in the 190's when I workout. I also have experienced blackouts upon standing and simptoms of low blood pressure when sitting,( nausia, lightheadedness and i've fainted once). I've had an EKG and everything was normal, the cardiologist I saw blew everything off and said i need to drink more water, I'm an athlete so i know all about the importance of hydrating and i drink plenty of water, ( I even upped my intake just in case and nothing has changed). I'm going back to the cardiologist but i have very little confidence this will be resolved before field hockey season  starts, so please if you have any leads i could follow they would be much appreciated, thank you

[Jerry_NJ]

At your age a HR nearing 200 when a maximum physical output should not be dangerous, but it should not be held there for a long time either.  A resting HR of 100 is also high, but not dangerous, maybe not even unusual for a teenage person, I don't know.

The fainting isn't good, but that could be the low BP.  I'd think it reasonable to "pressure" your cardiologist to be very definite about what HR you should be able to run to when engaged in athletics, and to give advice on your BP and measures to increase it if that is needed.

[ks1982]

It really isn't all that uncommon to get lightheaded if you stand up too quickly especially if you have low blood pressure to begin with (which is possible especially if you are athletic).   My blood pressure runs low just naturally and I've gotten lightheaded and fainted once or twice when I've stood up.  
Another thing that you may want to look into is the effect of hormones on your heart rhythm.  

[is_something_wrong]

I would guess you have low blood pressure. If that's the case, your heart will often try to compensate by over-increasing your heart rate.

You're writing you feel symptoms of low BP, have you measured your BP while having symptoms?

High heart rate with normal blood pressure is not dangerous. It rarely causes damage to the heart, high blood pressure (pumping against high resistance) is far worse. Anyway, if you have low blood pressure (confirmed by doctor) and there is no obvious cause, drinking more water (and salts) is a good idea, as your cardiologist said.

[grendslori]

One thing that I always have an issue with is why doctors tell people to drink more water to stay hydrated when it takes more than just that to do that. You NEED to eat protein along with taking the fluids in order to stay hydrated as it is the protein that helps your body to hold onto that water.