Sex on 1st day of period.

Tarra • I have one son. I had a miscarriage on November 26, 2014. I was 14 weeks and 5 days pregnant, but the baby was only measuring 10 weeks and 5 days.
My period started saturday, very light, almost just spotty. My bf and I had sex on Saturday, what are the chances that I could have gotten pregnant? My period was very light the whole time, and it only lasted until Tuesday. It usually lasts 5 or more days.