I found a great site for all kinds of accurate information on cats (and dogs, too) when my son adopted a new preemie kitten. She seemed to go into heat about 5 or 6 months later and this site has so much info, alphabetically listed, that I was able to answer many of the questions that I was finding myself wondering.
The site is: vetinfo.com
This is some of the simple information that I needed to know and got it from the site with this as the last part of the address: /catnorm.html (of course it begins with www. )

Gestation - 63 days Estrous Cycle - 15 to 21 days Estrus - 4 days
So this would let you know that if your stray came to you pregnant, then she would have had the kittens within 63 days or by the end of February, so I don't think she was pregnant. If she does go outside sometimes and has not been fixed, then she could have gotten pregnant during the time you have had her. There are places that you can get free or near free spaying done. If you are interested, I can give you more info that I have found.
Good luck. I hope this helps and that you enjoy the site. It is a good site run by vets.

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Trish