I, and several colleagues, had the same a couple of years ago. You're right, the letter itself causes enough stress for them to connect you to the ECG as you open the envelope! We all did the treadmill without problem and without any licence implications, beyond them saying they might want another one in a few years. In my case, my AME says I've always had the 'blip', as do many people.
I suspect that occasionally the cardiac consultants, who get paid a fortune for looking at the routine ECGs, try to justify their existence by calling in a batch of unusual traces.
Although they probably do 'catch' the odd person on the stress ECG, statistics would seem to show that most people get through OK, so don't worry too much (and I know how difficult that can be). Are they letting you continue flying until the ECG? If so, another indicator that it isn't a big deal.