I had an aortic valve replacement in July 08 and my heart fibrillated for a week after. In fact I was a couple of hours away from a cardioversion when the heart went back to normal rhythm.

This May when I visited the gym after a mild viral infection my heart rate went straight up to 160 whilst I was walking (warming up) on the treadmill, apparently it is quite common after an infection. Up until then I had been able to jog at 8.5 Km/hr for 5 - 10 min and the rate would plateau at around 130 -135. However, I have had 3 further instances since, the last one today on the treadmill at the end of a gym session (incline 10% and 5.5 Km/hr so not particilarly strenuous).

The heart always goes back to normal fairly quickly although it is quite sensitive for a short time (15-30 min) and can go into tachycardia again quite easily even if I am only walking.
Tachycardia always occurs on the treadmill and I have not experienced it on the rower or bike (maybe they are less strenuous).

After the first two events the cardiologist was not concerned and suggested not wearing the monitor, but I find it very disconcerting when it happens so I am considering asking my GP for another referral.

Good luck Rita!

(Many thanks for the blog Adam, it has been an excellent source of information for me whilst I have been recovering)