If a condom breaks, it's not safe sex -- more like "started out safe sex, ended up unsafe sex." You were about as protected by the broken condom as if you did the exact same actions in the same order without him wearing a condom, so you have to think accordingly.
It sounds like you're saying that he hadn't ejaculated before he pulled out. But when a guy gets erect, pre-ejaculatory fluid comes out of his urethra. Pre-ejaculatory fluid doesn't always have sperm in it, but if the guy had ejaculated some time earlier, it can. There are millions of sperm in one ejaculation...if some that were hanging around in his urethra hitched a ride in his pre-ejaculatory fluid and out into the condom, and then got into your fluids, the sperm can live in your body a few days. Given that the sex was just after your period ended, the timing would be favorable to produce pregnancy.
Such a scenario is obviously not the easiest way to get pregnant. But you're seeing some light spotting, and unfortunately it's not impossible. It would probably be worthwhile to do a home pregnancy test in a couple of weeks.