If I can feel kitten move, how far along is she?

I believe my cat Alice is pregnant. I believe my male cat from outside snuck in the house and they got busy. He is often let in by my little boy. I'm not sure how long ago it was. Her belly looks a little bigger and her nipples are a little bigger and pinker. I also believe that I felt babies moving in there today. How far along could she be. When should I expect kittens? I haven't had the chance to take her to the vet yet.