Back in the day, I recall that I had to spend an entire day keeping doctor & dentist appointments before I got my orders to report for Basic. One of those appointments was a treadmill run while connected to a machine.
I wonder if they’ve relaxed those requirements to ensure fitness for training...
That would be a cardiac stress test, and they only do those if they find some underlying condition they need to evaluate. The doc probably saw something that concerned him on your EKG or heard something through his stethoscope that he wanted to check. The entry physical has never been very in-depth. Standard annual physical, plus a chest x-ray and EKG. I do think they are a little bit easier today with waivers for many conditions that would have been disqualifying 40 years ago. Sadly, this stuff happens.