Irregular period

Hello i am 19 years old me and my boyfriend had sex on dec. 28 2016 and my period is supposed to start jan. 12-17. We made sure that we used a condom and it stayed on and did not break from what my boyfriend said. Its getting near the 17th and my mom is getting suspicious i had sex again. My last period cycle was nov. 12 and dec. 14 2016. its generaly about 30-32 days from start to start. Can any one help me understand periods?