How long have you had her? When did the rescue get her? If she is preggers, and nipples are just starting now, and you cant feel babies yet, it would be 3 or 4 weeks along, into the 2nd month you can feel them, and movement. I doubt a breeding at 3 months, i have seen 6 and 8 months, but a full heat at 3 is unlikely, so either she isnt prego or she is about 8 months, how old do you think she is?
Good luck, i hope she isnt, if she is aborts are pretty safe, here we abort up until guarded viability, so i think you have time. I have had young mothers, and it just isnt right, they get scared, and can die, and kittens die, it happens, happened to me not long ago if you remember. Good luck, cant wait for the update on the sweet girl!! Also, she is old enough for a safe spay, she could have been done prior to coming to you, it is starting to be standard for rescues to spay before letting them go, and that is good, it is just too risky, good luck again!
I prefer my people to wear fur..
Gonzaga's Lady Perdita "Lady" CKC Yellow Lab
Adoptee Gucci super fuzzbutt
Adoptee Prada crazy dog extrodinaire
Tickles Tortie cat the owns my daughter