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If she delivered at the time that you have described, she is probably in heat again and he is ready to mate.
You can solve this problem by having them spayed and neutered right away. If you don't, you are going to have kittens again.
There are low cost spay/neuter programs in almost every city. You should call your humane society to get a list of participating facilities.
Let me know if you have more questions.
Keep in mind that cats will go in and out of heat until they are bred. So, within a matter of weeks, she can go right back into heat if she did not breed with the last heat.
The best way to keep them apart is to give each their own space. You can use stacking baby gates in doorways (so they can't jump over) or give each their own room. The baby gates work because they can smell one another and don't lose their familiarity.
You can also try a homeopathic remedy that we have used with great success. It eases anxiety. It's called homeopet and you can find it at www.homeopet.com.
I hope this helps more!