Thanks ladies for your answers!
you cannot be preg in this case. if your cycle is normal 28 days long, then you got to have sex midway between your two periods...and then test when your next period is already late. the order has to be : period > sex > missed period > preg test. it cannot be any other way to get preg
You can't be pregnant and still be having a normal period. You need to have sex leading up to ovulation to increase your chances of conceiving and if you do conceive then, then you wouldn't have gotten your period a couple of weeks after. So the nausea etc you're feeling isn't because of pregnancy - it;s something else.
For future reference you need to test at least 2 weeks after intercourse or when you period is late.
The 'chance' that you would have ANY symptoms after only 3 weeks is next to impossible. If you are nauseous, it's not from pregnancy. As for weird feelings in your body ... again, there would be absolutely no way to experience any feelings when a grain of rice would be huge next to a 3 week old conceived egg. When should you test for a pregnancy? About a month after a missed period.