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She is not pregnant. She could not have ovulated this time. If so, she would have a heavy period later. If she doesn't have periods for 3 months, she should see an OB & GYN to have exam, a blood test, a hormonal challenge test, and imagining studies.
For sure, take another test in 3 days. If it is negative, she is not pregnant.
It can sometimes happen because of stress, quick weight change, so forth. But there are some diseases which cause no periods such as overactive thyroid, polycystic ovary syndrome, brain tumor producing prolactin, some medications, and so forth.
No. She may be having a period. Can you rate my answer positively? You can continue to ask questions without additional cost after rating.