I started my period on the 25/09/2015. i have 4/5 day cycle. i had sex on the on 03-04/10/2015. could i have gotten pregnant during this time?

Maybe: Life is not exact. If a woman has sex and later is found to be pregnant, then she got pregnant shortly after having sex. That's how it works, but there is no exactness to it. That is why having sex with only one guy gets rid of the need for an exact date. An ultrasound dating scan done at 8-11 weeks of pregnancy can give a good estimate of the start of the pregnancy, plus or minus 1 week.