After getting a perianal abscess drained out there is itching in the anus while passing stool. Is it a fistula? My doc said for check up after a month

Possible. If there is not ongoing drainage from the incision site, then it is possible. If drainage continues, may be from that. Either way, hard to check on your own (!) so better to call dr & get sooner re-check due to concerning symptoms.
Not necessarily. Can be simple irritation from everything going on. Itching isn't the usual symptom for fistulas.