I discovered that my cat was pregnant only a week before she give birth to kittens.
What behaviors or signs could have indicated to me sooner that she was pregnant?
Heat Cycles Cease
An intact female cat will experience heats every 1-3 weeks during breeding season. According to VCA Animal Hospitals
The breeding season for cats will vary according to geographic and environmental factors such as temperature and the number of daylight hours. In the Northern Hemisphere, female cats usually cycle from January until the late fall. Cats that live in more tropical regions or that mainly live indoors may cycle all year round.
Signs that your cat is in heat are mostly behavioral. They become much more attention seeking than normal, and will raise their rear end up while walking or being stroked. They also have a unique (LOUD) vocal cry during this time.
Any intact female cat of kitten bearing age will have regular heats (during breeding season), and if it does not your cat could be pregnant or have a pseudo-pregnancy (false pregnancy). The cat should see a vet.
Pinking (Change in Nipples)
The nipples of a pregnant cat become swollen and rosier in color about three weeks after mating. This can be difficult for inexperienced owners to determine, and some cats will experience this just from the heat cycle.
Appetite Increases
A pregnant cat will eat much more than normal (she needs enough calories to make new cats). This may be difficult to determine if the cat has been stray/feral for some time before you start caring for her, because cats in those situations tend to be undernourished anyways.
Abdominal Changes/Weight Gain
A pregnant cat will gain weight rapidly, but it will not be spread evenly around her body, instead it will be mostly in her belly. In later stages of pregnancy it's easier to see because when she lays on her side she will have a bulgy belly (the extra mass is solid and doesn't slide around like fat will).
Palpitation
Veterinarians and experienced breeders are able to feel the fetuses at a very young age, but attempting this unsupervised is not recommended as you can injure the babies.