I had very low blood pressure since morning till now/but i feel my heart is beating very fast and very cold, what's wrong..

Get an ekg : If your heart is truly beating fast then you need to get an ekg to find out what the rhythm is. I suggest you go to your local emergency room or urgent care center at once and be checked out. Your symptoms could represent a lot of things, some serious and some functional such as a panic attack or hyperventilation.

Need to get cheked: Please see your doc or visit urgent care center or er you might be in atrial fib sign of low BP and fast heart rate is indicative of need of attention and evaluation.

Volume depletion: Anytime blood pressure drops and heart rate picks up, in someone who is only nineteen years old, the likelihood is that you have a depleted volume. This could be from too much exercise and not enough fluid replacement , or even from too much loss from heavy menstrual flow.