After my cat’s spay, a scab next to the wound turned red; is it a shave injury from the vet?

A scab beside the spay wound, if it was a broken wound from the day of surgery, may be a nick from pre-op shaving or during surgery; if it appeared gradually after, it may be a skin infection from the cat rubbing or licking against the E-collar.

First watch whether the cat licks it; switch to a larger, softer protective collar and disinfect and medicate the wound. If there is no clear improvement after medication, take the cat to a hospital to check the wound and adjust treatment.