Thank you for your thoughtful reply, els!
My doctor called me today with my bloodwork results. She said my thyroid is completely berserk - my TSH levels are undetectable, which means my thyroid is way overactive. This could be causing my rapid, pounding pulse, and even my diarrhea!
I've got to say that hearing something as innocuous as this - Grave's Disease or whatever - is actually music to my ears. It's rather benign and treatable!
She's doing more bloodwork, but wants me to still get another cardiac work-up, EKG etc. Hopefully it's just this thyroid business.