I am 18 years old, i have been on birth control pills for over a year and a half. since starting them, my period has been very regular and predictable. my period ended exactly 1 week ago, but now i am bleeding again, dark red just like my normal period. i did have "unprotected sex" in those 7 days but again, I am on birth control and he always pulled out. however, i have been on antibiotics for strep throat for a week, so could that cancel my birth control/ cause this random bleeding? please help me, this is so out of the ordinary for my body. could i be pregnant?