Hii i understand your problem. Iam glad that a cardiologist has ruled out a possible cardiac issue. This is certainly looking like an anxiety disorder and what you experiencing at a sudden given moment must be a panic attack which mimics a heart attack.
Firstly do not worry, its happening due to certain neuro chemicals imbalance that happens in the brain. You need to see a psychiatrist in person to identify the cause of this anxiety going so high. Some medications will help you in this same.
In middle of lockdown if seeing a doctor is not possible and medicine support not avail at least have a good lifestyle.
Next Steps
Relax yourself, sleep well 7-8 hrs, eatwell some exercise or walk daily. Do some activity that you like and feel happy doing atleast once in the day.
Divert negative thoughts and counter it with positive thoughts. Some pranayam or deep breathing will help you.
Health Tips
No drugs/ substance use.