is the side effect of the the pill seen sometimes in some women,
but the success of the pill can only judged by negative pregnancy
the side effects usually last for one to two days but menstrual irregularities may occur in the following few cycles.
there is 1% of failure rate of pill means there is 1% chance of becoming pregnant.
you don't worry about it, just do urine pregnancy in case of missed period that will rule out the possibility of pregnancy.