How could I have a heart attack after a normal nuclear exercise test?

Without having seen the results of your test, it's difficult to comment on it. But the likelihood is that it really was fine. Here's the most probable scenario for what happened: You had a moderately sized atherosclerotic plaque in the inner wall of the right coronary artery, the artery that runs down to the bottom of your heart. That plaque probably wasn't big enough to compromise blood flow and thus cause angina, or to make a worrisome signal appear on your nuclear exercise scan. But even a small plaque can rupture and, when it did, it caused a blood clot to form that blocked off the artery, causing your heart attack.

I know this scenario is scary, because it means a heart attack can occur in anyone at any time, even someone able to run miles the day before. While true, your chances of having a heart attack are much lower than average if you can exercise without symptoms or if you have a negative exercise test.