Cat bullying other

So two of four cats: oldest female approximately 5 years old, spayed, (Brittney) and second oldest cat, approximately 3 years old, male and neutered (Butters).

Brittney semi-recently started going outside through the doggy door at night. Butters didn’t seem to like that and would act aggressive through our glass door. Being a responsible owner, I took Brittney to the vet to get updated vaccines, she got an abscess, and is now on the mend after treatment and stitches out.

Butters has now started being aggressive to her throughout the day, chasing her and getting some scratches in, occasionally she pees or poops.

They’ve been around each other since he was four months old when we adopted him. They’ll still take treats near each other, eat near each other, sleep near each other at times. How do I address this behavior? I don’t know if Brittney’s defensiveness and growling starts it, or if Butters is just flat out being a bully.

Unfortunately due to the recent abscess treatment (around $2,000 total), I am unable to take them to the vet for a behavior consultation. US, adult female.

TLDR: neutered male now bullying spayed female that was sick, previously no issues. Please ask any questions!