It's probably pustules from a bacterial skin infecttion. The fact that the antibiotic hasn't helped, just means that a different antibiotic needs to be tried. With bacterial infections, it's a matter of finding the right antibiotic that the bacteria involved is sensitive to. You can tell your vet this antibiotic isn't working and request a different one. I've had good success clearing up bacterial infections in my rabbits with azithromycin, but it can cause digestive upset for some rabbits, and whether or not it will be effective just depends on the bacteria involved. Ideally a culture and sensitivity test of the bacteria is best, to know exactly what bacteria it is and what antibiotic will work. It will increase costs, but it may become necessary if trying a different antibiotic doesn't work.
You may also want to ask your vet about opening up the pustules to keep the pus cleaned out, and applying a rabbit safe antibacterial ointment(eg. plain triple antibiotic ointment) on the area. I know of other owners that have found these pustules on their rabbits genitals, and just opening them up and getting the pus out seemed to be enough to clear up the problem. But this would only possibly be effective if it's localized just to that area like isolated pimples, and isn't affecting anything else internally. If it is just localized to those few pimples on the skin, then opening them up and getting the pus out may be enough, and changing to a different systemic antibiotic may not be necessary.
https://wagwalking.com/rabbit/condition/bacterial-skin-infection