The CFA site on reproduction in no longer there. They want people to buy the disk. Here is another source, and I will do a bit more searching. Some questions however, especially if there's an unusual situation, will require a vet's answer. Regardless of whether you think she's in season or not, any possibility of her getting out is very risky.
It is not unusual for cats to come into heat about 4 weeks after kittens
are born. A few cats will have an estrus period, in which they may become
pregnant if bred, at about 7 to 10 days after kittens are born. Most cats
will actually have suppression of estrus during the period they are
producing milk for the kittens and then come into heat a couple of weeks
after the kittens are weaned. So it is possible for a cat to have kittens
within 2.5 to 3 months after their last litter of kittens is born, but it
It might be unusual, but my female came into heat twice while nursing kittens.
Here's a good link for you: