I had Cushing's and I was tachy... Cushing's effects the adrenal glands and your hormones there. I had to take florinef which is a aldosterone replacement. Most of the time, Cushing's patients have high bp but there are the odd ones like me that have the low BP so it can happen.
I would ask them to scan your adrenals, and get more cortisol tests. Now, my adrenals looked postitively brilliantly unremakable on MRI and CT (with contrast) and my reports were glowingly normal... yet when the surgeon went in there, those puppies were HUGE. Again, I know this has happened to many of my friends (I only hang out with the best). Better yet, my post-op CTs still say they are normal. Now that is a feat! :D

I suggest more endocrine work ups...