We were adopted by a stray about 3 weeks ago. Took her to vet, where I found out she was pregnant, vet thought about halfway along. But, that was a week ago, and she had a kitten last night about 9pm. It's now 1pm, and she hasn't had any more. I think I can feel kittens still in her abdomen. When should I be concerned if she hasn't had another birth?