Well, that is obviously difficult to say from here. I mean, you have been to the ED and your doctor's office, had ekg, probably blood tests, ect, and they still have not seen anything. Typically your presentation says, dehydration! But, apparently this isn't the case as you stated. Another culprit could be atrial fibrilation with rapid ventricular response, but they should have been able to see this on the ekg, unless it is somewhat sporatic and it just didn't show at that moment. Hopefully the halter test will find any abnormality in your heart function.
Now, with all that said obviously something is wrong, right? Your blood pressure is too low which is causing your heart rate
to increase to try to compensate. So why is it too low? As previously mentioned, typically you would see dehydration or something like atrial fib, but apparently you don't suffer from those. Other things to consider would be heart defects such as with the valves, causing the heart to not pump effectively, thus reducing the amount of blood pumped at any given time, which would reduce the pressure. You could also consider abnormal kidney
function. Your kidneys may be forcing your to remove excess amounts of fluid, but then that would typically be seen as dehyration. Bacterial infection could lead to abnomal dilation of blood vessels as would nerve damage
from things like diabetes
. Lastly, reduced cardiac function can be caused by a pulmonary embolus.
My guess is that this is related to the heart rhythm in one way or another however.
I hope this has been helpful and good luck.