The wonderfully dependable clue - ONLY useful in a first time pregnant cat - is that the nipples will turn bubble gum pink once they are 21 days along...typical gestation period is about 67 days.
I used this key change to tell when my cats were pregnant, when I had a registered breeding program some years ago.
I agree with the rest, best to have her examined by your vet, and to get her spayed if not alreayd pregnant. Allowing her to be bred by ??in the neighborhood exposes her to felv or fiv, and depending on her age, some risks do exist in pregnancy/delivery at too young an age.