Dear honey,
you think to prepare for Civil Services Exam, then what prevents you. Just Go ahead! Also you can do M.Sc., Physics. My cousin got only B.Sc., Chemistry in college during his admission when he interested much to study Physics. But after finishing his B.Sc., Chemistry, he approached university and got exempted to appear for B.Sc., Physics and got his second B.Sc., - so, with interest one can do anything. Go ahead! Wish u all success!