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The sex 6 days and 2 days prior to expected menses is highly improbable to cause
pregnancy, however it is not impossible.
The egg release in woman takes place only once in each menstrual cycle and that is around 14 days prior to the next expected menses. Considering the life of the egg and lives of man's sperms, a week around this day is the probable period for happening of pregnancy.
Usually it takes around 8-10 days for any symptoms of pregnancy to appear. Hence, just 3 days is too early for any symptoms.
As it is you have got the menses. So very unlikely that you are pregnant.
However, if the bleeding is too little, associated with pain in abdomen, there is a possibility of pregnancy somewhere else than the
uterine cavity - may be tubes. This can be sorted out by the Obstetrician only.
Have you done pregnancy test on urine. That should come positive in any sort of pregnancy.
Your symptoms can be due to some anxiousness also. Better see your Obstetrician to resolve the dilemma.