Antibiotics and kariva birth control

Patient: Hello, i’m on my 3rd month of kariva birth control and in the 3rd week of my 2nd month i was prescribed cephilexin for a staph strep throat the 4th week i had my period. it was all normal. then i had unprotected sex in the first week of my 3rd pack (but he pulled out). now i am worried that the cephilexin i was taking could have interfered with the birth control and i could be pregnant.does having my period and then starting a fresh pack of pills protect me “fully” from pregnancy again? does cephilexin interfere with kariva birth control?im so scared. please help me!!!