hi, i am a female aged 17 and i do not have a period as i know that could cause some symptoms. I constantly feel weak all the time, like i have no energy. I have a piercing headache everyday which wakes me up every night and impacts my daily life, especially at school as i cant concentrate on anything. I am always tired like i cant get enough sleep and i am always thirsty even though i drink about 3 liters of water a day. I urinate a lot and my vision is becoming blurred. I also get a racing heart beat when i stand up for 2 minutes or longer and feel very lightheaded like i am going to faint. i don't know what's wrong, any suggestions?