My mom thinks I am pregnant?

Starting friday after school my mom made me take care of my 1 yr old cousin. I had to pick her up from daycare at 3:30 and my school lets out at 3:16 (but my mom and step dad took my car b/c i went to see my bf one day) Luckily my bf took me to get her. I have the baby all weekend. This is suppose to deter me from and pregnancy. Which is stupid b/c I am on birth control and do not want a baby. Ok. so I have a bf who is 19 and I am 17 but my mom thinks I am pregnant even though I am on birth control. I feel she is just upset at the fact she found out I was ually active. We made an agreemennt that if I got on birth control I would tell her when i lost my viginity. However I did not but I told my older sis and my bf's mom (who I am much more comfortable with) however they did not no I was on birth control. So out of worry they told my mom. What should I do to convince my mom I am not pregnant. My bf wont even come to my house while my mom is there anymore. lol Oh and i also thinks she is made b/c he is at my house when she is at work.