She can only get pregnant from sex around her ovulation. Ovulation happens about 14 days BEFORE her next period. Sex from about 5 days before ovulation to the day after can get her pregnant.

Nothing is guaranteed. It is not a case of having sex and getting pregnant. Only about a third of woman will get pregnant from sex at the right time.

An early results test can be accurate from about 3 days before her missed period and is almost guaranteed to be accurate 7 days after her missed period.

Ask her to explain her cycles and how her body works to you. If she does not know, it is a good idea to educate yourself on it.

Wait, if she had her period, then she is not pregnant.
Now if I misread that sorry, but I quote you here, to show you where I got confused:

There are some pregnancy tests out there that are very sensitive and can detect the pregnancy hormone up to 5 days before a missed period. Some examples are Clearblue and First Response. Others advise you to wait until you miss a period or 3-5 days after your missed period. If you still think you are pregnant, yet the test shows up negative, the best thing to do is schedule an appointment with your Gyn/Doctor, who can do a blood pregnancy test (it is more accurate and more sensitive to the pregnancy hormone, HcG) and transvaginal ultrasound (they can detect early pregnancies, from about the 5th week, if I am not mistaken).