Um just because she had her period doesn't mean there's no chance of her not being pregnant =.=
I'm no expert but its still possible even if its a overnight thing..
No, if she had her period she's not pregnant. This is because the eggs are expired and would not be hospitable to producing a child. If she did get pregnant a few days before that, she would have a miscarriage as the egg would be a hostile environment.
However, there is a period of 'spotting' after impregnation in-which the female bleeds lightly and over a period of time as the egg grows and the sperm is implanted. This is not easily confused for a regular period, and any female (except maybe one on strong birth control pills who just started them) should be able to tell the difference.
Another thing to take into account is the period of ovulation. Figure out how long her cycle is, when she last had her period, and plug it in to this:
http://www.ovulation-calendar.net/
That will tell you roughly when having sex would have the highest probability of a successful impregnation and you can therefore avoid those dates. They usually last anywhere from 3 days to a week in which you can expect her to be the most fertile.
There's also a ton of signs to look for in pregnancy. One of the most common is heartburn. If your lady has heartburn for the first time ever, then you should most definitely research it. Other signs include frequent trips to the bathroom for urination, over exhaustion, swollen or tender breasts, unscheduled mood swings, odd food cravings, aversion to well-liked foods, and vomiting usually in the morning or before bed.
While not all of those signs will directly inform you of whether or not she is pregnant, if you notice 3+ of them in the same day, you should invest in a pregnancy test. (Get clearblue, they are the most trustable, and you get 2 for $10.) Do not use a pregnancy test before 1-2 weeks. The results will be unreliable within the first week since hormone levels won't generally spike until the end of the first week.
Also note that semen can live within the vagina for up to 72 hours. If you have sex, and cum inside, there is a chance for her to become pregnant over the next 3 days without ANY intercourse, which makes predicting the date of pregnancy difficult. Rinsing out (douching) inside the vagina will help reduce this, and flush out excess semen.
All in all, over the past 6ish months, my fiancee and I have had at least 5 pregnancy scares (roughly one every month

). We've finally decided to stop worrying and just let nature take its course. If you're not ready for a child, use a condom, or abstain from intercourse. There are still plenty of enjoyable activities for you to engage in with your partner without the unwanted risk of an unexpected pregnancy.
Good luck

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EDIT: After reading your previous thread, I would say there is a 100% chance she is not pregnant and never was. Precum only contains semen if you have ejaculated recently and not urinated. If you have urinated twice, then there will be no semen in your precum. (This is because precum itself does not contain semen, but instead takes the semen from the urethra as it exists the penis). So, no worries whatsoever

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