If you were at risk for a heart attack your stress test would most likely have revealed this. I think the fact your stress test was fine, coupled with your very young age, puts you at very low risk of heart attack.
Here's the typical symptons of a heart attack
Uncomfortable pressure, fullness, squeezing or pain in the center of the chest lasting more than a few minutes.
Pain spreading to the shoulders, neck or arms
Chest discomfort with lightheadedness, fainting, sweating, nausea or shortness of breath.
Not all of these signs occur in every attack. Sometimes they go away and return. If some occur, get help fast.
Heart attacks for people in their 20s are very rare events. It's understandable that you feel some anxiety about the death of your brother. If you haven't already seen one, I would go to a cardiologist and get checked out. I would definitely tell him/her your fears. . Put everything on the table and have a full, frank conversation. The odds are greatly in your favor that it is not a heart problem; you just need some reassurance. The doctor should be able address all of your concerns, and you should feel better for it.
Good luck!!