M.D.
I would go on line www.kellymom.com has information about what is safe. Or Google the name of the antibiotic and breastfeeding. I would not want to double dose my baby. And I would consider probiotics for your child as well (yogurt with live and active cultures, for instance).
Typically they don't presribe antibiotics for the flu (they don't help with viruses, which is what causes the flu).