Anal sex on period, pregnancy risks?

I have around 31 day menstrual cycles & am on day 5 now but its coming to an end.                            Yesterday on day 4, my boyfriend and I had anal sex, being on my period I was wearing as tampon. He did not ejaculate inside me.                                                                                                                                  I'm not sure when exactly my next ovulation will be. I'm thinking around the 20th-24th but am not sure.     With all of this in place, if semen did drip into my vagina (which I don't think did, because of the tampon & what I felt) and it last for usually 2-3 days, how likely is it for me to become pregnant? I'm hoping not that likely.