Pregnant?

Ok. So ive been on birth control for three weeks, and am supposed to start my period in one day. Well, i fell asleep and forgot to take my pill last night, which is also the samw night my boyfriend and i decided to try doing things without a condom. How likely is it that i could get or be pregnant? (Ive heard that its unlikely to get pregnant so close to your menstration date, but im curious and am paranoid)