Why Do I Feel Weak After Doing Some Work?

hi i im 18 years old and feel so week all the time if i do anything by the end of the day my whole intire body is throbing and hurts so much i get dizzy and headachs i just feel old and week all the time i cant even stand for a while because i get lightheaded and i get alot of hot flashes and is always hot can you help me to know why i know something wrong because i never use to feel like this. thank you