In all honesty, it is sometimes impossible to tell immediately. In some cases, signs of being in heat abruptly stop, which can be an indication. If the mating is successful and the pregnancy progresses, your cat will gain weight, have an increased appetite, and have swollen nipples. Some cats develop gastrointestinal signs, such as vomiting and diarrhea early in pregnancy. If the mating does not lead to pregnancy, your cat may experience a pseudopregnancy with similar signs or return to her usual estrus cycle.