How is a pelvic abscess after a hysterectomy treated?

Depends. The location and cause of the abscess is of importance and so is the reason for your hysterectomy. However, the basic approach is the same. The infection needs to be drained in some way, and antibiotics are needed but can't do the job alone is most cases. See your gyn surgeon if you suspect this problem.
Abscesses. These are often drained surgically and then treated with strong antibiotics.