Hey dear.. First of all i want to tell you dont take so much wory about your condition.. Because this is very common after horrible dreams.. Nightmares may come from your activity during day,if any horrible movie you had seen, or read bad news on tv or somewhere else..or sleep deprivation, some disorders, The situation aroud you may be responsible for it.. During nightmares, due to the fear your cardio vascular system will work more which causes high blood pressure,due to this condition you feel tired and heavy heart..For more diagnosis u should consult doctor near by you.. Live happily and healthy will help u to come out from nightmares..