How do I tell my female cat is pregnant?

My cat Mamas shes about a year or a little more. She isn't really into other cats I see her chasing them out of our yard and being mean (hissing, spitting, growling, and eventually attacking them.) I have caught her hanging out with the same cats in alley which I know are all males but shes never been pregnant in her life. She is showing signs of bitterness toward her best friend our dogs and she's cranky toward us as well just not as much as the dogs. What are the symtoms of the very beginning of a cats pregnancy? How long is the gestation? How do they behave when first pregnant? If she did has sex would you be able to tell by looking at her vagina? I have seen my rabbits vaginas with a stretch or tear/rip on the bottom or top.