Hi... If you have gone through all medical check ups and all reports have come normal, then it suggests that your problem is more psychological.
If you're already going through some issues that are stressing you out or something has happened in the past that you are unable to overcome mentally then I suggest you start taking Counselling Sessions from a Psychologist online/offline on Weekly Basis.
Next Steps
Start a Breathing Technique (Deep Breathing, Alternate Nostril Breathing) daily for 20-25 min morning and evening. Start any Physical Exercise/Sports daily for 40-45 minutes.
Health Tips
To Seek my Services Online you can Google me or visit my Website (https://gunjanmaithil.wixsite.com/therapy)