120/80 is considered the "normal" BP; young women may be lower, 110/70.
80/50 is low, but there are some possible reasons why you're getting that reading. First, the cuff apparatus at the pharmacy may not be accurate. Second, it's not entirely One Size Fits All ... and if your arm is large, just as for someone who is very skinny ... the readings will be off by a certain percentage. That being said, even if it's out by 10%, your BP would still be only 90/55.
Try going to a different pharmacy, and check your BP several times there too, or go to a walk-in BP clinic or public health unit, and have a nurse check your pressure accurately with a proper BP cuff and stethoscope.
I read through your journal, and you've mentioned that you've had some troubles undereating, and also not really drinking enough fluids. In this hot weather, it's imperative that you keep your fluid intake high. Caffeine may not be the best thing, because it dilates the blood vessels temporarily, which can make the blood pressure lower. Crash dieting can also cause the blood pressure to drop. That's why it's so important to keep your calories and FAT intake up.
.. If you're sweating a lot, you need to make sure to keep levels of potassium up ... consider using light salt, or half salt, which contains potassium chloride mixed with regular salt.
If your BP is way low for you, you'll feel dizzy and lightheaded, and possibly quite weak. If you're not experiencing those symptoms, I'd say everything's ok. Otherwise, if you are having symptoms, it would be good to have a health professional check it out.
Doreen