Those sound like panic attacks to be honest. They can come out of nowhere. We can get a surge of stress hormones. Your cortisol will shoot up and man, it feels terrible. Yes, it feels like you are going to die. They did a physical on you, I assume. Went through your symptoms and checked things. Your symptoms do sound like anxiety related. That's not a bad thing. It can happen to anyone.
Have you ever heard of DBT therapy? It is distress tolerance therapy. Is there any warning that it may happen? (the panic attack or anxiety attack or attack, whatever you want to call it). Things that DBT covers is deep breathing. There is something called square breathing, breath in for 4, hold for 4, breath out for 4, hold 4, repeat. This slows the amygdala and resets us. Opening the fists firmly keeps blood flowing. If you are home, immediately doing something like jumping jacks, running in place, push ups (on the floor or up against the wall) is a reset. Ice in the hand or even ice cold water splashed on the face is a good reset. Grounding. This is where you pause and think about your feet on the ground. Feel the ground against your shoe. Look around and see what you can see. Listen to the sounds. Get in tune with these things and it grounds you in the moment. Have a mantra, 'this will pass. I'm okay," for example.
Start to journal to get things out. See if you have hidden stressors. Just get more aware.
And really, medication can be a great thing. For many with anxiety, it keeps them going. Zoloft may not be the drug for you but remember it takes time. It takes 6 weeks to reach full therapeutic effect if you are on the right dose. And sometimes you have to increase dosages especially with zoloft which often takes a higher dose. And for panic, you usually need a bit higher dose. SSRI's have start up side effects where you may feel funky or even an increase in anxiety or mood symptoms early but that goes away.
Would you consider a talk therapist?