A long time ago I was told by a doctor to drink coffee to bring mine up, but then coffee isnt the best thing to be advising.
This really sounds like you are having a problem with your sugar. Unfortunately it doesn't always show up on a blood test. That would explain why the hospital wasn't able to detect a problem. You blood sugar level had already stabilized by the time you got there. Keep a candy bar in your purse and the next time you feel your blood pressure drop like this eat your candy bar right away. I am reccomending you to keep it in your purse incase you are not at home when this happens. Make sure you finish the candy bar. If this helps then you need to see your regular doctor and explain exactly what happened. I really hope this helps.
Randy
Try drinking an electrolyte solution or Have a salty snack and a glass of water when you start to feel like that. For the rest of the time make sure you are having a good diet with plenty of fresh fruit and vegetables and salad etc. Also take an Omega 3 tablet. All the best.