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She might be having Atrial fibrillation
which is causing the irregular heart beats. She must avoid taking tension and getting anxious. The medicine given by the Doctor should keep her heart rate
in control. If the heart rate is irregular and usually above 100/min it could cause clots in the heart which could be dangerous for the brain. You may need to speak to the Cardiologist about the risks. Take her to the Cardiologist if she feels dizzy, excessive heart beating, weakness immediately.