@chai2,
As I said, I only had tachycardia once (140?), and it was my good fortune to be working not far from my boss, a credential heavy m.d., who immediately guessed it was a reaction to, pardon me, constipation. With my diet, which good or bad or indifferent, has left me through the years not dealing with that very much, I've never had another tachy episode. Or maybe that was coincidence - though it seems not. Whatever my blatant faults, I'm generally not constipated.
So - I'd keep notes on what you are doing/eating/drinking around when this stuff happens. I believe you that you weren't particular stressed, and somewhat roll my eyes re therapy advice.