I would feed again on the 18th to give the antibiotic time to be fully excreted.
I lost a boa to that last year, except the infection presented as an eye infection/swelling rather than a neck swelling because of the infection's location. So, we treated for eye issues instead of using an all-around course of antibiotics.They think it may have been a scrape inside his body from the claws of the f/t rat and then it got infected.