Mother approval of BC!! Help!

My boyfriend and I have occasional sex about once every two weeks due to a long distance reationship. We're both very in love and dedicated to one another; however, I still am not on birth control and we usually use the "pull out" method (not too smart, I know). 
I can't talk to my mom about having sex, because my family believes in abstinence until marriage.. They gave me a purity ring at 16 (but it was too late 😳).
Is using the excuse of awful period cramps acceptable enough for my mother to be okay with me getting on birth control? Also, I'm 18!!