Yes, you could be pregnant. A lot of people think you cannot get pregnant during your period, but that is not true. I would recommend, in the future, using plenty of lubrication when using a condom, as this prevents the condom from breaking. Also, make sure your boyfriend is wearing a condom that fits
him well (a condom that is too small is more likely to break). Lastlym it wouldn't hurt to periodically check the condom to make sure it is still intact. It may not add much to the mood, but better safe than sorry.
A brief explanation: A newly-conceived embryo doesn't need the lining that the mother has made (yet), so being on your period doesn't mean you can't get pregnant. In fact, a lot of women who conceive shortly before their period still have their normal period because the embryo doesn't yet need the tissue of the mother.