If you started your period than you're not pregnant like the last poster said your fertile days are sometime around 10-14 days after the first day of your last period. Sperm can live up to five days but that still wouldn't be around your fertile time.
I am trying to get pregnant, so my Obgyn told me to have sex on days 11-17 every other day. Day 1 is the first day that you start your period. This only works if your periods are regular. I hope this helps. So I don't think you got pregnant on the same day that you started your period. Plus don't forget that sperm can live up to 5 days in side you.