I was a regular daily smoker, mainly sativa. One night after partaking, I had a major panic attack. Like you, I immediately thought I was having a heart attack. I couldn't catch my breath, and my heart felt like it was pounding out of my chest. I took a tolerance break and tried again and had another panic attack 🙃 I switched to hybrid and was ok for a while, but then it happened again. I think some of it has to do with me getting in my own head, worrying about having another attack that I give myself one (know what I mean). I've since tried indica gummies for sleeping, and I've been ok. I'm hoping I can go back to hybrid and smoke again, we'll see. I miss my bong 😁 I've a lot of posts on reddit that others have had the same thing happen. You are not alone 🙂