The appearance and distribution of these patches suggest a contact dermatitis, that is, an allergic reaction to something that came in contact with your skin. This is not the pattern you'd get from a reaction to an internal antibiotic. I don't think these are infected, so I doubt that the antibiotic has been relevant one way or the other. Having said that, I'm not at all sure what would cause such a contact reaction in such an unusual distribution (it isn't poison ivy, for instance.) You should see a dermatologist as soon as you can. I have little doubt that this can be treated and that it will leave no scars, since contact dermatitis, no matter how blistery, is just in the epidermis, so when it goes away it's gone.
Congratulations on your newborn.
Best.
Dr. Rockoff