Hello,
All chest pain needs to be evaluated by a doctor as a precaution, but what is important is to determine what other symptoms your sister may be experiencing.
Besides the feeling of lightheadedness, did she experience any other symptoms.
Did she over exert herself in exercise, or even by lifting something heavy?
Are her breasts tender and sore, perhaps a result of PMS?
A consideration is that she could have costochondritis which is an inflammation of the cartilage that connects a rib to the breastbone. It causes sharp pain in the costosternal joint where your ribs and breastbone are joined by rubbery cartilage. Pain caused by costochondritis may mimic that of a heart attack or other heart conditions.
Here is some info, let me know if you have questions:
http://www.mayoclinic.com/health/costochondritis/DS00626
http://www.mayoclinic.com/health/chest-pain/DS00016
This is a very helpful tool:
http://www.mayoclinic.com/health/symptom-checker/DS00671/SYMPTOM=43C61DFB%2D2A5D%2D9994%2DEAEA50FC8AAE8FF3&TAB=Chest%2520pain
Please let me know if you have any questions.
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