Palpitation or fast heart beat as you have mentioned is most commonly associated with
Thyroid problems like Hyperthyroidism or if Thyroxine treatment dose is high for Hypothyroidism.. Since ur Echo is normal and ECG shows only sinus tachycardia, u need not worry.
Next Steps
Check
TSH levels and consult Endocrinologist to adjust the Thyroxine dose. If palpitations is frequent then Tablet. Propranolol 10 mg twice daily can started for 3-4 weeks.
Health Tips
Don’t be anxious and try to start Practices like Yoga which will help to resolve anxiety issue