You really should not worry.. Your chances of being pregnant are one in a zabillion. (or maybe not quite that high)
but keep in mind that you're on the pill, and if you're on your period then your egg is no longer fertile and has been dropped.
Your highest chance of getting pregnant is the time opposite your period. (the middle of your non-period time). Which is when you are most fertile. You started your period so your chances of even being fertile or able to get pregnant are slim.
However, this is still not an excuse for unprotected sex... Don't forget about std's.