If my girlfriend took plan b on september 4th got her period about a week later, now she is having spotting that started on the 18th.... any ideas?

Plan b effects: Plan b works by disrupting ovulation and temporarily messes up hormone levels. Irregular bleeding after is not surprising. Hopefully you have stopped having unprotected intercourse. When she schedules a visit to start her birth control she can also get tested for stds and discuss her spotting.

Could be from Plan B: Emergency contraception symptoms can include spotting or bleeding, changes in menstrual flow or your period may be late or early.