Can nitrofurantoin cause breakthrough bleeding when on oral contraceptives? if so, how long will it last, and how long should i use extra protection?

Probably not: Nitrofurantoin gives very low blood levels; it's concentrated in the urine and therefore good for UTI, but not enough in blood to explain vaginal bleeding. Antibiotic interference with hormonal contraception is much rarer than media and websites suggest. Any unexplained vaginal bleeding requires medical attention. (At your age, STDs are common causes.) See your doctor soon.