I've had this also, but not due to A-Fib, it's due to my ANS (autonomic nervous system) failure. A-Fib is about the only arrhythmia monitors haven't picked up for me.
Sometimes my HR will be 150, 170 or 200+ and my bp will be 90/60 or lower; it can be a mixed or crossed signal from the brain, to the heart, to the ANS/CNS.
if you read in the Dysautonmia (Autonomic Dysfunction) forum here, you'll find many of us ANS patients are like this - some are dx with POTS, OI, PAF etc
http://www.medhelp.org/health_pages/Neurological-Disorders/Autonomic-Dysfunction--Dysautonomia-Index-Page/show/857?cid=196
Find a good cardiologist and get a cardiac workup to see if they can find what's going on, if it's Afib it can be treated, ANS issues are bit trickier and may take some time for a diagnoses or even be mis-diagnosed.