Ive been on birth control pills for about 3 weeks now (sprintec) me and my partner had unprotected sex on the 23rd of this month and he did cum inside me (we was caught up in the moment) im supposed to come on my period around the 26th...is it a small possibility im pregnant...? If so...how late can i take a plan b?

My mom or sister never talked to me about sex or birth control so i dont know that much about it...please help