Anyone with a pheochromocytoma?

Hi-
I am a 32 y/o female who has had Tachycardia pretty much all of my life. I saw a pediatric Cardiologist at around 16 and he told me it was nothing to worry about.
My heart rate was always above 100 and could can get up to over 100 upon exretion.
In January I began passing out. I was referred to a Cardiologist and was diagnosed with a Tachycardia induced Cardiomyopathy. My EF is approx 40% now.
After many tests, and many meds I was referred to my Endocrinologist. He has ran many tests. Which has led him to believe I have a Pheochromocytoma.
I have had
- A 24hr urine = Increased Catecholemine levels
- A Clonidine Suppression Test = My levels only suppressed 10% after 3 hours
- A CT scan of my Adrenals = showed no mass

I am scheduled for an MIBG Scan next wk, in which he told me it is possible it will not show up on there.
My symptoms seem to be getting worse. I have had several spike in BP while on Coreg and my Heart rate is around 100 but during an episode has gotten to 150. When this happens I sweat profusely and get really weak.
My question is, are there differential diagnosises that could create these results?
I am so worn down from all of this and just wonder what happens if the tumor is not seen on the MIBG scan???