If she's just getting fat, it would show over her entire body. A vet can tell after 20 days if she is pregnant. Another clue would be vomiting after 3-4 weeks. At about 5 weeks, the cat's nipples will enlarge and turn a bright pink. Then her belly will begin to expand. Your cat will begin to look for a safe place to give birth. You can prepare a box for her, lining it with a soft blanket or towel or shredded paper and placing it in an area of low or no foot traffic.