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Since the EKG, has been done and your doctor has concluded that your cardiac parameters are within normal limits, there may no reason to worry about cardiac issues.
The pain could be due to musculoskeletal issues or due to breast lesions (benign tumors like fibroadenoma, abscess, fat necrosis, etc).
In case the pain increases with direct pressure on the chest wall, it may indicate a musculoskeletal issue. A chest X ray might rule out any bone lesions. A breast ultrasound may be used to diagnose any breast lesions.
You may consult with your doctor or your OB-GYN, for proper diagnosis and advice.