Not sure where this belongs..

Not sure exactly... But my boyfriend that I've been with for 9 months refuses to wear a condom, but does not want a kid right now. He's onto me all the time about birth control. I started birth control at the beginning if August... A week later I had emergency wisdom teeth extraction and was put on antibiotics. Well I haven't had my period since the beginning of August. Took tests an they came up negative. Well my boyfriend says I better not be pregnant cause I had plenty of time to get on birth control... How do I explain to him in a way that pregnancy prevention takes two?