No one is "too young" for cardiac problems. They just are not as common in younger people. But SLE does have a nasty reputation for causing an inflammation in the tissues that line the heart. They become swollen and inflamed and it causes serious chest pain.
I'd suggest you see another physican pronto! If the second opinion is that it is not the cardiac muscle causing the pain, ask if it is possible that what you are suffering from is in actuality chest wall pain. It feels just as frightening as cardiac pain, but will respond very favorably to immediately ceasing the offending activity, and resting quietly until the pain subsides.
Any chest pain that is accompanied by feelings of nausea, cold sweats, diziness, tingling in the left arm or jaw, or chest pain that radiates is a serious call for attention, and should not be ignored.