I'm 21 years old. I had intercourse, one day before my period. The protection broke and my boyfriend isn't sure if he ejaculated inside me or not. Is it possible that this could lead to pregnancy? He said that that could just be the semen, but he isn't sure. Should I take a contraceptive pill one day before my period?

Yes, you should be worried. Any time there is penetration pregnancy is a possibility.

Taking a regular contraceptive pill will not prevent pregnancy. That is what the morning after pill is designed for and must be taken within 3 days of the encounter. The earlier the higher chance of it working.