Almost Every Single Time I Stand Up I Everything Goes

Almost every single time I stand up I everything goes blank and I can’t see for 8-10 seconds it’s go so bad before I have fell and hit the floor I’ve went to the doctor and they had did ekg,tsh ,CBC w/diff ,cmp and a holter monitor and everything came back normal I go the the cardiologist the 25th because my doctor just wants to make 200% sure that there is nothing wrong with my heart and of not she’s sending me to a neurologist to see if there is somthing wrong with my brain .

I’m not exactly sure whats going on with me but but it’s keeping me up st night because I’m scared that I might have some form of epilepsy or somthing