Hello, 

I am a 15 year old girl who weights around 110 lbs and is 5'3 (normal, I am assuming).

Over the course of several months, I've been noticing extreme dizziness/lightheadedness when standing after I had been lying down. It can get really bad and it's hard to stand up for too long. My room is in the basement and I also struggle going up the stairs without getting short of breath. Not to mention, after lying down and then standing, I also have blurred vision for about 15-20 seconds and then my eyes adjust and everything seems to be somewhat alright. I've been experiencing lots of migraines or terrible headaches which I used to never get. I have a problem sleeping as well. I can fall asleep easy and I am extremely tired, but I can't stay asleep without taking medicine to keep me asleep. I've always been a light sleeper so I am not worried about the not getting a lot of sleep, but mostly the drained energy/fatigue because I am tired more than the average person I would like to believe. 

I measured my bpm when sitting and it's around 65-75 (normal range), but when I stand, it spikes to around 115. 

I'm also a runner (cross country and track). I have went on several runs where I had to stop because I thought I was going to fall over. The other day, I went to a fitness center where my mom works and ran on the treadmill. I only went a mile and I found myself drifting and almost falling over. 

I just want to know what's wrong with me -serious or not. I need an explanation of some sort. I drink lots of water and water only. I don't like coffee or tea and I have a pretty regular diet.