hi, im a 26 year old female, while standing up today on a train, i felf fine before hand, suddenly i became weak, cold sweats nauseous, i closed my eyes to concentrate on my breathing and i lost my vision for about 5minutes maybe more, it came back slowly, but felt tired and weak post episode?? is that normal? i have felf faint previously in the last year but sustained loss of vision was traumatic for me?? my GP reckons i was dehydrated but i drink plenty??