In terms of trying to date how far along she is, at this young age, it is safe to assume it is her first pregnancy. Without fail, once I learned this, exactly on day 21, their nipples will become visibily pinker - a bubblegum pink, and seem larger. It isn't so on day 20, but is on day 21, and then remains this color throughout the rest of the pregnancy.
It's not as accurate with subsequent pregnancies, but is for the first. Not only I, but all the breeders I knew, used this to confirm a cat was pregnant.
Gayef has a wonderfull overall sticky on pregnancy and labor, might be a good review