At this early stage there is no way to tell if she is pregnant. However, if she was in heat and she got outside chances are she is. The earliest signs of pregnancy are small behavior changes like longer sleeping periods, mood swings and better appetite. After about three weeks there is a more reddish (raspberry) or brownish tone on the nipples accompanied by a tendency to swell. After a few more weeks there will be rounding in the body and soon after the kicking of the kittens inside will be able to be seen and felt.
Your best bet is to take the cat to the vet for an exam. If she is pregnant then you can be sure that all is OK with her.
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