how she have more babies

hey ok im confused i had a mother rat with her babies, babies r 3 1/2 weeks old and there hasnt been a male with her since a week befor she droped 14 babies, now this morning i go to feed her and babies and she is sitting on 15 pinks:O
Does anyone have any clue on what happened did she breed with one of her boys or?

Female rats, and mice, can store sperm, and have several litters from one breeding.

I have a female mouse here that has done it 3 times. And another that has done it once.
Often, the stored sperm results in a smaller litter than typical.

It's also possible she bred with one of her sons, but they're so young, i couldn't imagine.