went to the ER and was told they didnt know what was wrong
In your case, until you've been checked out thoroughly (hopefully by a good cardiologist), you don't even know if you actually had a heart attack. Other conditions (many minor) can mimic heart attack symptoms. (That doesn't mean you should ever ignore the symptoms, though!) Do be sure the doctor knows what kind of life style you have (including backpacking).
I had an acquaintance who started dayhiking a few weeks after open heart surgery. Of course he couldn't carry a backpack until everything healed up, but at least he could get out on the trail with a waist pack!
Please try not to worry too much until you have a diagnosis! I know that's hard (if not impossible), but too much worry will just exacerbate the symptoms or could (if what you had wasn't a heart attack) bring on a real attack!
Good luck to you, and please keep us informed!