I had my stress ECG yesterday - I was already producing 40% more power that they would expect for someone my size when they stopped me (I did tell her that I cycled). They had to get me off the bike with me protesting that I could still go a lot faster and wasn´t at all tired yet. Anyway, it was quite interesting to see what the heart does - in my case, an extra beat every so often.
I´m now hooked up to a 24hour ECG which is a PITA as I can´t take a shower with it on.

The doctor said it was fine to do a triathlon on Sunday, and wished me luck, so that´s put my mind at rest. For anyone else who has any of these problems, it´s worth being checked out, even if just to reassure you that you are fine.