I just want to see if there is nothing to worry about...any similar experiences out there?
The side effects can be divided in two parts: those due to injection of the radioactive compound and those due to the treadmill stress test. Your father did not have any side effects form the exercise phase, but side effects from the the treadmill can be chest pain, irregular heart rate and rarely, death have been reported during treadmill stress test. There should have been a doctor supervising the test that is the appropriate procedure, so it may have been a dotor who answered questions and not the tech.
It is possible your father had side effects from the injected diagnostic agent. One can be allergic to the agent, etc. and side effects can include: chest pain/pressure, dizziness, headache, nausea, dizziness, shortness of breath, or a warm and flushed feeling. Some patients experience a burning or stinging sensation at their IV site.
He had the pressure during the injection and as a result of the injection, he did not have pain during the test.