Keep in mind this is an academic definition of a term that is strongly considered derogatory.

Literally, a "bitch" is a lactating female canine. Generally speaking lactating dogs, or dogs who have just delivered a litter are a bit touchy. They have a tendancy to growl and snap when others come near them. They become fiercely protective of their pups.

When humans emulate or seem to simulate this type of behavior, they are metaphorically referred to as bitchy.