Female receptivity

Question
A small stray black cat adopted us three weeks ago when she was pregnant. She had her kittens March 29 or 30 as near as I can tell without seeing her kittens. Yesterday, another stray who uses our yard as a respite looked like he mated with her. Can this little female get pregnant again so soon? A very unofficial source told me that she wouldn't be receptive this soon unless her kittens were dead. Is there merit to this answer? When can I take her in to be spayed?  

Answer
Most likely she won't get pregnant until she is done nursing, but you never know so she could get pregnant right away.  You have to wait until she is done weaning her kittens to get her spayed.  The best thing to do is to get her to come inside the house.