you could lie and explain to her that you need it to regulate your periods or just be completely honest and tell her you are sexually active but you want to be safe responsible and adult about it. Either one is effective
I am not sure what MA is, but don't worry most of us feel like this at some time or another in our lives. I know it sounds silly, but go and talk to your parents, tell them you need some encouragement rather than put downs, they are probably not doing it to hurt you, they just dont realise. If you can't, go and visit a doctor they can get you counselling, or even talk to teacher at school. Don't suffer in silence, it only makes things worse.
They don't want you to be so scared that you don't buy the cereal. Same with the Frankenberry. I don't buy that one coz Al Franken is a schmuck.