Answer: Another strain of bacteria must have caused your own sickness
Explanation:
Bacteria are microorganisms that can cause diseases. They exist in different strain (i.e different subtype, though of the same specie).
So, the strain of bacteria that caused illnesses in you and your sister must have been different, and because different strains have unique metabolic machinery, enzymes, and other proteins; the same antibiotics that was effective in curing your sister's bacterial infection would rather be ineffective in your own case.
Thus, your doctor must prescribe a different antibiotic tailored to your bacteria strain