Sounds like gerd .. there’s a good over counter one starts with a o but don’t know spelling ..I take that .. happens too me every fast also I have tachycardia Wich fasting I found out according too my tests slows my heart rate too a normal rate Wich is fantastic for me
I agree that it could be digestive/reflux pain but there’s a possibility your electrolytes (potassium specifically) could be imbalanced which can cause cardiac arrhythmia. Highly advise talking to your doctor just to be safe!