Hi there,
May God bless youu for taking her in.
If she was outdoors, it is very possible she is pregnant again. Cats can go back in heat when the babies are only two weeks old.
If you feel motion she is at least three weeks along soo it is too late to termiinate the pregnancy
Yes it is safe for her to have another litter so soon but all the babies may not survive.
Since she is negative for FELV and F IV your cats cannot get anything from her.
I would let her give birth, keep her in and have her spayed when her litter is five weeks old.
Please let me know how things are going.
Warmest best wishes,
Terri
Feline Healthcare Expert
Expert in feline health and behavior. 20 years experience with cats.