Period started a day after unprotected sex?

Rebecca
Okay, ladies...
My man and I had unprotected sex two days ago on a Sunday and he...Of course...Came inside me. I wasn't mad or upset the night was great! I would've bought plan B either way and I knew my period was going to happen either Sunday or Monday. However, I DID start my period on Monday afternoon. 
But has anyone ever gotten pregnant from having unprotected sex the day before a period? I know some people will say "you can get pregnant at anytime" or "no it's not possible to get pregnant" 
Did a lot of research and I haven't seen any posts about women who have gotten pregnant from this happening. 
I need some advice!