My girlfriend missed a day of bc earlier in the month and now she is spotting blood what does this mean?

Missed pills : Missing birth control pills will trigger menstrual spotting and decrease the effectiveness of birth control. If this is a continued problem for her she should consider using a different form of birth control.

It could be: Break through bleeding if it is not the anticipated time for her period. Usually missing one day of birth control will not make a significant difference - but for each day missed the efficary of her oral contraceptive decreases to the point that it is like she is not using protection if she misses a week of her birth control pills.