rabbits can become pregnant right after birth, and can mate very quickly.. if you have allowed the buck anywhere near her even for a moment, she could have become pregnant again.
babies (esp boys) needed to be separated at 8 weeks old, as around 12 they're already becoming fertile and could impregant a female, I have heard of stories happening earlier. hence why 8 weeks is a good benchmark.
have also heard of rabbits mating through bars if they're big enough.....hence why I'd always suggest keeping them far far away.
she could be pregnant again, or she could be unwell (bloat). I'd suggest getting her to a vet.
has dad been neutered yet?