150s tachy, normal BP, episode brought on by exertion. what's happening? (elderly)

[catandbear]

A woman in her 80s, hx hypertension but mostly in good health. She was walking for around 40 minutes. She was feeling weak and had to take breaks on her way home.  When she arrived home she had an episode of tachycardia (158) (stated pulse felt "irregular, but not very irregular") with BP (144/71)  (not sure if it started while walking). She had no chest pain, only palpitations and general weakness.
She took 1 mg digoxin PO and eventually the episode resolved.  Had two more similar episodes in the past year: tachy with normal BP brought on by exertion. EKG was normal the next day.
She's a dear family member who doesn't live in the states and her doctor didn't believe her when she described the episode. Luckily digoxin is OTC in her country, but she would not want to take it prophylactically due to potential side effects.
Any ideas what this could be???
Thank you in advance!!!!
PS She reports that she is normally on the brady side (HR 60s) with high BP (systolic around 140s, but up to 200)