With small “hot spots” I’ve had good results by mixing triple antibiotic ointment with a little benzocaine (cold sore or toothache remedy) to numb the itch a bit. The side benefit is it tastes extra nasty so they tend to lick a lot less.
Since she’s a rescue she may just be having adjustment problems manifesting as a nervous need to lick her backside. Lots of extra love & reassurances will help the most over time, but you may have to physically keep her from doing it long enough to break the nervous pattern. In that case you may help her most by using a cone around her neck so her hiney gets time to heal.