Its highly unlikly to have gotten pregnant right before your period. Even if you did ovulate during that time the egg needs up to 10 days to implant into the lining of your uterus! Your most likely to get pregnant 14 days before your period. But its alwas wise to take a pregnancy test 2 weeks after any unprotected sex! I hope this helps!
If you took it within 72 hours, you should be fine and since you started your period you should be in the clear. The morning after pill messes with your hormones. I know I have taken it before on two seperate occasions, and each time it made me start my period. But I know some women don't get their periods with it, it effects everyone differently. I wouldn't be too worried about it. If you are still worried in afew weeks take a pregnancy test but I am sure you are fine.