It is only unlikely for some women. Lots of people get pregnant when on their period. Especially when you are on the pill. This week is the only week that your pill isn't keeping you from ovulating. Sperm can live in your body for 5-7 days. Some say 3-5, some say 3-5, but when I researched it, I found 5-7. So, even if you don't have an egg ready to be fertilized when you have sex, you may 7 days after!
You can get pregnant anytime that you have unprotected sex, or protected sex if the condom breaks. It is against the "ideal woman's cycle" that is all over the place (the whole ovulating on the 14th day of your cycle chart.) but, that is only for some women, and the rest of us are totally different. So, if you don't want to become pregnant then don't have unprotected sex, anytime. Not even on your period.