Weed does cause some heart-racing after people smoke it, but it's likely not so much the weed itself as the action of smoking that does that. For people with unhealthy hearts because of coronary artery disease or a propensity for arrhythmias, that can indeed be risky, but for people with healthy hearts, it's probably no more stress than running a couple of laps. You've got the extra added stressor of Adderall, which is a stimulant that can also speed up your heart rate and make you more prone to panic attacks.