Is it possible you have a heart rhythm anomaly that is skewing the reading? There are many kinds of arrhythmias and tachycardias you could have and not all of them are indications of poor cardiovascular conditioning. My boyfriend had one that actually would stop his heart for a little while, then when it got going again it felt like it was beating very fast. It was an electrical thing and they zapped the nerve in an out-patient procedure and now he's good as new.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 192.5lbs, press 75lbs and deadlift 210lbs