constant light headedness

i have a sub-neumonic chest lung infection , recently given up smoking marijuana and cigarettes , i am extrememly underweight , my skin is pale , i am always tired , and every second of every day i have this wierd light headed feeling that is really hard to explain , i have had several doctors examine me , i have had a CT brain scan , lung x rays , the works , but nothing is showing up , my full blood count is essentially normal and it has been a good few weeks that i have had this feeling in my head and it has shown no signs of letting up , i am almost positive that it will never go away it is so constant it is driving me insane .. i cant live like this. please help because noone else can . im stuk like this. :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-( :-(