so if it was heavy with large blood clots and dark blood, than there's no way i could be pregnant
Very...in fact, you can't get pregnant the day before your period starts, because by then the egg released during ovulation is no longer viable, and your "period" is your body's way of flushing out the dead egg and the uterine lining.
BUT...with that said...you can get pregnant a day before you THINK your period is due...because it's possible to ovulate late, which means your period wouldn't actually be due when you expect it to be, so it's possible to get pregnant at any point during your cycle because you cannot guarantee that your cycle will be regular every single month....
but, if you had unprotected sex and then your period started the next day on time, all the sperm and the egg would be flushed out with the blood, and there would be no live sperm left by the time your body releases another egg, so your chances would be almost nothing.